A lot depends on whether you are happy with square format. For portraits, it has the advantage that you don't have to turn the camera for portrait format, but you may end up wanting to crop to portrait, which might mean you're effectively getting 12 645 shots from a roll rather than 16! If you want square and longer lenses, then AFAIK roughly there are 3 choices, in increasing order of cost Bronica SQ, Hasselblad 500 and Mamiya 6 (a rangefinder with fantastic lenses). TLRs ruled out because they only come with "normal" lenses. I'd guess for a good specimen only the Bronnie would fit the budget (the M6 would be 2-3 times budget!).
If you want 645 and portrait format, there are issues. It's notable there is a range of Fuji 645 cameras, some autofocus, which are portrait format by default, and have good lenses (mostly or all non-interhcnageable?). But I think these mostly pan out as wide rather than long.
The Mamiya 645 and Bronica ETRs are common 645 SLRs (4:3 ratio). (You can get a 645 back for the Bronica SQ as well.) You'd not want waist level finders, as it's (literally) a pain in the neck to use a WLF on its side for portrait format. You'd want a prism, which means you can get a metered prism. The earlier Mamiya 645s do not have interchangeable backs ("inserts" does not mean the same thing; you can't change film easily until it's been fully wound on). There has been a later Mamiya 645 Pro with interchangeable backs in the classifieds. (Oh, and there's a later Mamiya 645 AF with lots of auotmation, metering etc, and the option of an expensive digital back...
@robhooley167 has one.) Any of these would be within budget. I don't know much about the Contax 645, but I suspect my eyeballs would fall out when seeing the prices! I ought to know more about the Pentax 645, which I think might be naturally prism vf (
@Mr Bump has one?) .
67 is a format that a lot of people like; fairly close to "publication formats" like 8*10 so you don't lose much in cropping. Three obvious choices that I know of, all by Mamiya. But you can rule out the Mamiya 7 straight away as even more expensive than the 6. Then there's the RB67 and the RZ67. These are big and heavy and well-built, we joke that you could hammer a fence post in between shots! I think the difference is that the RB is more mechanical and the RZ more electronic, ie more to fail? The advantage of both is that the backs can be rotated for landscape or portrait, so you can use a WLF. Very good lenses, maybe on the cusp of your budget? Again there's a Pentax 67 I know nothing about (and maybe a 68).
Then 69, getting back to the "full frame" ratio 2:3. Not too many of these beyond the folders, but there is the legendary Fuji "Texas Leica" rangefinder (GW690?) which a few folk on here have had.
I guess 612 and 617 are not of much interest to a portrait photographer!
Apologies if I've got any of that wrong (someone will correct me, no doubt), and I'm sure there are other options in each category (wrong word, it's all more fluid than suggested here).
So, as I see it the choices do stem from how much you like square. Is there a chance you could borrow one to experiment?
There will be revolution by Simon, on Flickr
Mamiya-C330f by Andy, on Flickr
GlovesB1-sepia by Andy, on Flickr
Medium format vs 35mm by Kyle, on Flickr
Mamiya 645 Cameras by Kyle, on Flickr
Normal Lens Comparison by Kyle, on Flickr
Cuppa by Kyle, on Flickr
Jess... by Kyle, on Flickr
Jess by Kyle, on Flickr
Jess by Kyle, on Flickr
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