Whole point of interchangeable lens cameras is they have interchangeable lenses.
Means you have two issues/targets.
1/ Camera Body
2/ Lens.
Of the two, the lens tends to be the more crucial, and you're in a world where if you have Interchangeable Lenses, they tend to be the expensive bit that holds value. This either means that to get it all inside budget, you are probably going to have to skimp on one or t'other.
Incumbents in the market are Nikon and Canon. This means that a) that's what there's most of about, also that's what there's most interchangeable lenses available for.
Oh-Kay... NIKON, have been using the F-Mount for its lenses since some-time in the late 1950's, and it was one of their marketing tags that 'any' Nikon F-mount lens would fit any F-Mount camera, throughout that time. It's not 'quite' true, but close enough. Means that you can fit film-only era, manual focus lenses on a modern auto-focus DSLR without an adaptor or issue. (give or take).
Means that there is a plethora of F-Mount compatible lenses out there to choose from, and many of them great lenses and not exorbitantly priced. Which makes it the bench-mark system to look at.
B~U~T you need to think lenses NOT 'Camera'.
And work from there.
In the world of legacy lenses there are some fantastic offerings at amazing prices out there. Rather denting the merits of the Nikon F-Mount, I have a rather good Ziess 50, that is the 'normal angle' lens on my old M42 film camera. It's worth about £10 on e-bay! A genuine Nikkor F-mount 50 might still only cost £50 or so, but gives you an idea of the quality of legacy lenses you can get for so little money. But they tend not to be auto-focus, and there can be some fun and games with legacy lenses not letting a camera meter through them.
This brings up the entire world of issues with legacy lenses and adaptors, starting with old lens fungus, and sticky aparture blades etc... but..
Point is, if just ONE half decent lens is likely to consume half or more of your budget.... it wont leave a lot for the camera to plonk it on... or whole point of interchangeable lens cameras, getting other lenses you can plonk on the thing!
Hence even more reason to think LENS not CAMERA.
How important is being able to use the camera 'Point and Press' and having all the functions like auto-focus there, on demand every time How likely are you to want other lenses? (I'll say now, that as soon as you get an interchangeable lens camera, and can fit alternative lenses the compunction to try will be high?!! beware GAS, you have been warned!), and what sort of lens range is going to be the more useful/important to you?
On that, the 'normal' angle of view provided by a 50 on film or a 35 on DX is a pretty good start point. And another GAS warning the 'wow' factor of telephotos leads lot of folk to chase more of that, when going wider is often the more beneficial and more challenging, even before you find that wider lenses tend to be in shorter supply and more expensive...
But you already have a mega-zoom bridge..... which offers some ho-hum and more thunk, and suspicion that using an ICY camera you will find the wide-side the more wowing, and when using more telephoto be wowed by what they can do, and chasing longer focal lengths the f-no doesn't drop as much and they will seem to actually 'work'....
Which brings us back around; think LENS not CAMERA.
If you buy in-to say, Micro-Four-Thirds, from Sony or Panasonic ect, there are some great looking VFM outfits on offer, BUT there aren't so many lens options for the format, and the ones you will most likely want to try will by sods likely be the more expensive ones.
If you but into say, Nikon F... there are thousands of lenses available, and the prices of them can be brilliant.. but more if's.... most wont Auto-Focus, and depending on lens/camera compatibility, some still wot AF even though they may say on the lens they are an AF lens, others wont let the camera meter etc et etc....
So you need to work out what LENSes would be most useful first THEN work out from that what camera body and or system would be most helpful.
A few years ago, for daughter doing GCSE photo, and aghast at what she was doing with MY camers, we budgeted £200 to kit her up for Christmas., Out of that we got her a 2nd hand D3100 body and a 2nd hand DX35, as her main lens, and had change.. a little.. for a triupod. She could then use my lenses, AF and legacy Manual and adaptors on it... so £100 is maybe a bit on the skinny side... but.... If I was at it again, I would be strongly pulled towards the Nikon F-System, especially now they have finally introduced a 'new' ICY lens system and F-Mount lenses are likely to start popping up ar more reasonable prices 2nd hand... though 'more reasonable' is still very relative!
The D3100 we got for daughter was echoes with another we for for the O/H, with the kit 18-55 for about £150, maybe eight year ago. That is an 'entry level' DSLR and you might just eek something like that, or later D32oo or D333oo in your budget... And is as good a place to start as any. Its all for the learning after all, and if it proves the wrong choice, you should be able to flip it 2nd hand for little loss.... you have to start 'some-where' after all.... but.... its all somewhat speculative and a bit of a gamble.. and at a mere £100 budget... well, what have you got to loose really?
But think lens not camera..... and just what is the more important, stop thinking video resolution and mega pixies, start thinking angle of view and apertures, and nail it down as 'close' to right first time as you can.
On your budget, its going to pretty much be hobsons choce anyway, and its a case of getting as few compromises in the deal as you can, ad much as as many of your wish list... so pays your money and take your chances..
But think lenses first, pick a stem to build on, and dont expect it all at once.
As said, I would likely go the Nikon F-Route again, and still plugging away with my D32oo after a decade in digital, I wouldn't be too unhappy to go that way again, and again, priorarise getting the LENES I wanted obver how many mega-pixies or how flip-out-able the screen was on the camera body.... in fact, the D32oo, an entry level camera, probably just on your £100 budget threhold 2nd hand, lives in a bag with close oi £2K's worth of AF lenses for it,m to get the sort of 'wide' I had with my film cameras! It would be one to look out for.. B~U~T its just the beginning, not the be-all-and-end-all of the game... LENSES is what will make or break the deal here, and there the costs, even 2nd hand wail tend to make the price of the camera body tend to insignificance in the grander scheme... you have been warned and avoid GAS, all too easy to develop when you have a camera body you CAN fit alternative lenses and accessories to.
