Sirius Glass
Subscriber
I'm going to set the over/under prediction for this thread at 10 pages. EVERYBODY has an opinion (including me!). It's all good, though. Lots of opinions mean lots of things to consider.
I have 645, 6x6, 6x7, 6x8, and 6x9 medium format cameras. They each have something unique to offer. The joy of medium format is all of that glorious negative area. It's tempting to say go big or go home, but lots of medium format enlargers are limited to 6x7 negatives and smaller. Except for some old folders with primitive lenses and quirks, most of the 6x7 and larger formats will be out of your price range.
I'm partial to 6x7 if you can print it with your enlarger. I have several Bronica cameras, including a GS-1 system. Bronica cameras are workhorses with excellent leaf shutter lenses. Leaf shutter lenses will sync at any shutter speed, which is nice if you're working with flash/strobe systems very much (especially portraiture). They don't have the bellows of an RB67 or RZ67. They also don't focus as closely if that's important. A decent GS-1 body, with a 100mm lens, finder, and film back can be had for $500+.
I'm not a fan of 6x6 format. I know I'm in the minority here about that. I almost never print square. I prefer the proportions of 5x7, 8x10, 11x14, 16x20 images over square images when printed and framed. That's just me. If you're going to crop to a rectangle, you might as well shoot a rectangle unless you're a wedding photographer who doesn't want to rotate a large camera for portrait and landscape oriented shots. If you print a rectangle from 6x6, you're effectively throwing away the extra negative area over a 645 system. Still, that doesn't stop me from owning a few 6x6 cameras because they have other characteristics I like. TLRs are fun and I like the couple I own, but I wouldn't suggest one for someone's only MF camera.
The 645 format is the least expensive medium format to get into. There are lots of good examples. You could afford a Bronica ETR with multiple lenses, backs, and a non-metered finder for $500. I have a couple of them and they're fine systems. You can even get into autofocus 645 MF cameras, but probably not for $500.
I've suggested system cameras with interchangeable lenses, back, and finders. Those offer huge flexibility and if one part fails, it's sometimes cheaper to buy a replacement than get it repaired. Film backs in particular can wear to a point where they give uneven spacing to the film. Unless it's a rare camera, I just get another back for less than the cost of repairing the bad one.
I greatly prefer 6x6. 645 looks too much like 35mm on much less film. 645 does not allow much left for cropping. I also believe in cropping before photographing.