Before digital, B&W was usually the poor-man's choice. Colour photography has been about since the tag end of the 18oo's, it was just rather difficult, and most processed used an 'addative' system like trichrome, where three separate exposures, filtering Red green and blue had to be made, and then composited. I actually had a go at Trichrome last christmas with a roll of cheap B&W and compositing the results digitally in photoshop, which was interesting. However..
In the early 19oo's 'subtractive' colour processes that made three coincident exposures on seperately filtered 'layers' of emulsion a single piece of film, were pioneered, that were a little easier to manage, but still rather more complicated & expensive to mamage and make viewable pictures from.
Meanwhile, the 35mm film format was conceived in the 1920's, as a 'cheap' compact film format, able to use 35mm movie film, as the money in movies made the technical advances possible and made it cheaper and more available.
Monochrome... however remained popular, as it was still cheaper; especially in larger formats, particularly the 120 format, where negatives were large enough to 'contact' print without enlargement to make viewable possitive prints, and the process is 'cold' and reletively simple and DIYable.
Legacy of 35mm is somewhat perniciouse; and it's worth adding that its popularity was driven by being based around that common and reasonably inexpensive movie film. In the 1930's it was a format shunned by most photographers, as the smaller format and need for enlargement made it more expensive to obtain like for like image quality from it. It wasn't until the 1950's that the system gained much following, when war-time advances in manufacturing quality control, and the 'mass' market consumerisation started to see the cost/quality benefit improve; BUT major reason for shooting 35mm in those early days was simply COLOUR, you could shoot commonly available colour movie stock, and slide film, offered the advantage that you didn't need to have print's made to view, you could view the developed film stock in an iluminated magnifying viewer or on a projection screen. right up until the 80's and beyond, medium and large format remained teh favoured format of seriouse and proffessional photographers.
35mm B&W was actually something of a side step, and usually shot for cost or expedience rather than 'art'; as it was easier and quicker to home process and print, and at least up to neg stage, still cheaper than medium or larger formats, but right up until the end, pro's were still poopooing 35mm 'quality' for 'seriouse' reprographgics work.
Which may go someway to query some of your presumptions about film photography and B&W.. yup, its a magical thing to do, BUT; 35mm's forte was always 'cheap colour' not 'good' B&W.
Whilst 35mm SLR's, were a bit of a mongrel, and an awful lot MORE compromises for questionable merit. Main advantage was "What you See is What You Get"{or close enough} view finder composition; that advantage being core to the evolution of the interchangeable lens 'system' SLR, not having to change view finders or viewfinder lenses to match the taking lenses field of view, as had been niggle of interchangeable 35mm 'range-finder' cameras or 'Twin-Lens-Reflex' roll film cameras... did rather deminish the potential 35mm cameras had for being 'compact' though, and merti questionable if you dont use very many different lenses!
On which not; I have a rather nice Ziess Ikonta 'folding' 120 camera, with fixed {Damn woman's 'dusted' again, so I cant lay my hands on it! to check} 80mm lens. Slightly larger in outline than any of my 35mm cameras' but folded down it DOES far more comfortably slip into a jeans or jacket pocket, than any of the SLR's. That is a camera, that designed around the 120 format, offers the image quality 35mm users only aspired to, and where B&W was exploited to make it affordable, or for artistic merit.
So IF that is what you are looking for, that 'real' back to basics think about it photography; the Pentax & 50, nice camera as it may be, might be somewhat short changing yourself.
Personally, given your comments about commercial processing; I would be suggesting you use cheaper and cheaper to process colour film in that camera, apart from college photo class where they were popular and where B&W was the usual media to get folk doing it all DIY, colour was what that kind of camera was made for.
As to the home processing: When I got into home dev, my Gran actually revealed some family history and aparently her mother, in the 1920's & 30's was a keen ameteur, having started with an over the counter box brownie; and progressed to a 1/4 plate field camera; sending my gran, as a child, out to the farm to collect the eggs, so she could use the egg whites to make her own photographic emulsion in the kitchen! {and treat her to a propper farm house custard!}
I have to hand, Michael Langford's "the Dark-room hand book" and the historical processes he explains in the book are fantastic reading, even if you never have a go at home developing. BUT, revalation aught to be in how much is possible without any major investment in specialist equipment or a dedicated darkroom.
I still home-process B&W, as these days it ISN'T the 'cheaper' alternative, and hasn't for a very long time. By the 1990's standardised chemistry and 'Dry to Dry' minilab machines had brought the cost of C41 colour processing down to the point it was cheaper to get colour prints than B&W 'over the counter', and last few rolls of colour I've shot have been souped by ASDA for £2 a roll, as it's cheaper than getting the chemistry to do them at home!
Meanwhile, digital has made that the more normal viewing media; I could drag the enlarger out the loft and make prints still, but who would look at them? For the most part anything I shoot is going to eventually wind up digital, and Photo-shop has pretty much rendered the dark room redundant so I "kitchen sink" develop my exposed films, dry, and scan at home.
Kitchen sink developing doesn't take an awful lot of specialised kit, even to do colour or slide films.
A 'changing bag' to take exposed film out of the can and get it into a developing tank is useful, but Langford even explains how you can improvise an old black T-shirt or a dark towel and a draw for that, if you have to! so all you REALLY need is a developing tank and spiral, a kit of chemistry, a few empty pop bottles and something to time the 'soaks' with! You dont need a dedicated darkroom, or wet-room, or anything.
Making prints? Well, that gets a bit more tricky; but as my Gt Gran obviousely did, you dont need much more than some chemicals, paper and an ordinary domestic window. According to my Gran, her mum made contact prints from her 1/4 plate camera by holding them up to the kitchen window! and again, Langford provides a lot of suggestion on improvisations that can be made, and even how to build a cupboard darkroom or temprary 'bathroom' darkroom. For 35mm though you do really need an 'enlarger' to make conventional prints or decent viewing size.
So many colleges and schools clearing out 'redundant' dark room equipment these days, that's even less 'wanted' than old film cameras, you can set up a home dark room, these days for very little money, if you hunt ebay hard enough; and as said, you dont need to do any major home alterations to set up a temprary darkroom. I've made prints in my mother's kitchen whilst the family were asleep, doing little more than close the curtains and turn the lights off!
It could well be worth looking into. Experience of using a 35mm SLR, with a fixed prime, TBH is not a huge step from using a Digital SLR, beyond the likely faster prime, and small shift in DoF/perspective from the 'crop-factor' {or lesser lack of}, on one of the less automated exposure modes. Home processing, and possibly printing, is the next step to taking control of teh whole process from scene to image. And as said, a MF 120 roll-film camera, even a pretty basic fixed lens one, is more likely to offer you more of what you are really hoping for in terms of that astheatic and feel, taking that DoF/Perspective shift right off the scale from the small difference between Crop-Sensor DSLR & 35mm 'full-frame'.
But, it is a fun direction to explore. I still use my 35mm SLR's.. but I have to say, I still seem to shoot more with my little 35mm 'compacts' {Olympus XA2's and a Konica 'true focal length' C35} as I always have done, and in colour, exploiting the two facets of the format it was good at! 35mm SLR is a mongrel, from the beginning; and whilst sticking to the fixed 50 and B&W is a bit of accademic purity, that has merit, it was an over sold idea even in 35mm's hay-day, notion you needed an interchangeable lens system SLR even more so.
Advice? Tips? Buy a copy of Langford, and enjoy the journey through photographic evolution he takes you on and let it inspire you. Dont get hung up on that antiquated photo snobbery of 35mm System SLR's, fast primes and B&W. Chuck colour through it. Try some different lenses if they come up cheaply enough; Get a dev tank and a cheap scanner and have a go at home processing, see where that takes you.
IF you come back to what you are doing with the Pentax & 50 & B&W... hunt out a cheap 120 folder on e-bay; you should be able to find something pretty useful and useable for under £50.. a LOT under, if you hunt a bit! That does all you want to do with the 35mm SLR... but even better!
And remember, at the end of the day, it's the final result that matters. HOW you make a picture is of very little importance or relevence to very many people; its what they see when its 'done' that counts... so WHO is going to see the pictures you take? And what will they find interesting? 'Cos when all is done and dusted, dont matter how perfectly composed, how perfectly exposed, or how technically amazing a picture may be... if the SUBJECT doesn't interest anyone... why bother? No one will find it 'interesting'.
