I know some of what I'm going to say has probably been covered already, but here's what's happened to me recently.
Started with 7D. Saw the TS-E lenses, got intrigued, then saw the pricetags.
Then I started reading about Pentacon Six, and $100 tilt-adapters from P6-EOS. So i bought a few P6 lenses (and a few Kiev 88/Hassy 1000 lenses and a $20 adapter from them to P6-mount). A year or two later, and I've got pretty much the whole lens set, in Zeiss 50/80/120/180/300 and Soviet 30/65/80/90/250 (and a lot of double i've been meaning to sell, you want?).
After collecting all that glass and only using it on my 7D, it was at that point that the 5D mk3 got announced (with its pricetag). I calculated for the same price I could spend $200 on an EOS3, and take a least a few hundred rolls of film before I began to get anywhere near 5D3 price-territory, so an EOS3 i got.
Then I finally bit the bullet, and thought about getting a Pentacon Six to go with all that lovely glass, so I did. Plus a Kiev Prism (better than the pentacon prism), new bright fresnel/split-prism focussing screen, and prism mount-adapter. Probably came in at $200 including the shipping (would have been a lot less buying one with all the stuff together, rather than in bits and bobs).
Then I found a good bulk-deal on ebay. It was $400 for a Zeiss MC Sonnar 180/2.8 and MC Flektogon 50/4. Or at least, that's a good price for those two by themselves. So it effectively came with a free Zodiak 30mm fisheye, 90mm MC Volna, Kiev 60 (with bright screen), and Prism. (not a bad deal, all up, I reckon I could sell just the fisheye for $150, I already had one so it's a double).
And just recently (although I didn't mean to, wifey, honest), I 'accidentally' put in a bid on a Mamiya 645AF for just less than $400 shipped and won. $100 later for an extra film back, and a $20 P6-M645 adapter, and it works great, got my first roll of velvia in it now.
And the best part is that i don't need any glass for it, although the option's there (especially for the 35mm and 80mm f/1.9, no P6-equivalents of those). M645 also takes Pentax 67 lenses via adapter, in case a 45mm Takumar comes up cheap one day.
So what's the lessons for you?
Firstly, decide on a format Now. Square 6x6, or 6x45, (or 6x7)? Either way, Pentacon Six-Mount Zeiss glass is good, it can be used on P6, M645, Contax 645, and Pentax 645 by adapter. Pentax 67 can be used on all x645, but not P6 (although i'm working on my own adapter). Hasselblad glass is just as good (mostly the same Zeiss designs anyway), but you pay more for the Hassy mount and the Leaf shutter (which is useless unless you have a Hassy body anyway).
In P6, lenses to look out for are the Flektogon 50mm f/4 (the 65mm isn't that good
, apparently), Biometar 80/2.8 and 120/2.8, Sonnar 180/2.8 and 300/4.0. MC is better for flare, of course (some people think that the earlier 'zebra' versions are best because later on the East Germans had trouble getting good glass. I've got some of either type, I don't believe it). As always with 2nd-hand 20-60 year-old glass, how well it's been treated recently affects IQ more than how it left the factory last century.
If you then go the Square route, you can get a P6 or Kiev 60 (actually, the Kiev 60 is better built imho, like a Soviet Tank, although it weighs as much as one too, much heftier than the P6), or Kiev 88CM ('hasselbladski', because it's a cube-camera, if you prefer that look). Or a real hasselblad, but then bodies are more expensive and so is the glass.
There's also the Exakta 66, like a P6 but with aperture-coupled prism and Schneider glass. Probably costs more than a Hassy by the time you're done, $600 body and $700 kit-lens. (The lenses and bodies between P6 and E66 are almost always changeable to the other type though, so if you get a good deal on one you can buy the cheaper other)
And between the three main 6x45 versions:
- Mamiya is current and still in production (some co-branded as Phase One).
Any Sekor C or Sekor N lens dating back to the M645 can be used on any M645 camera, 1960 to today.
Any M645 AF lens can only be used on AF, AFD, DF etc cameras (or wide-open only on older versions).
Digital backs can only be used on AFD and DF cameras (ie, not the M645AF which i've got).
On the older side, the last 3 Manual Focus bodies are the Super, Pro, and ProTL, these are the better ones to get, fully interchangeable waist-level-finders and mid-roll-changeable film backs. There are some older versions that had cut-back features.
On the new side, the AF etc versions you can't change to a waist-level finder, finder is fixed. The 645AF I've got has one AF point, but i've got no AF lenses anyway (still, it's nice to have the option).
An MF body with metered prism and back might set you back $300-400.
An AF body might be $400-500.
An AFD or later body and you're at $700+
- Contax 645 is discontinued (but leaf/phase one still make Digital backs for them).
But the glass is to cream yourself over. Distagon 35mm and Planar 80mm f/2.0, baby. The 35mm Distagon is almost exactly the same as the ZE 21mm distagon, just bigger. Read any review of the 21mm and you'll see what I mean.
They also had a 'vacuum back' to hold the film flatter. Again: the price.
but yep, they're damned expensive, and rarer. A second-hand C645 body can still cost as much as a 7D. The AF is a '100k pixel sensor', sounds like Contrast-detect to me. So possible more accurate, probably still S___-slow.
- Pentax 645 I didn't investigate much, mostly because you can't change the back mid-roll to swap to a different film. But then I didn't realise that getting a film-back for an M645 would cost over $100 anyway, so i've only got 1 back anyway. They still make lenses, I don't know about second-hand film-body prices though.
There's other versions, like the 645 Hasselblads H1-H3 that can take film. Again, too expensive (although the glass is damn nice). H4 doesn't take film.
Rollei SL66 is a wonderful machine, with perfect Zeiss glass same as the Hassies and P6. But they're more collectors' pieces these days so prices are high. Lenses can't be adapted because they have no focussing helicoid (focussing was in the body, same as Mamiya RB&RZ.
Rollei 6008-series also good, also more expensive (although it's the only MF body that can meter with a waist-level finder), and glass-prices to match.
Zenza Bronica I know nothing about, although glass is apparently good.
tl;dr. Pentacon Six / Kiev 60 bodies and/or lenses. Or Mamiya 645 bodies and P6 lenses.
That's your best start on a budget.