I don't find the GA645 is that much fun to shoot with on a tripod. I'd only buy that camera to shoot it handheld. The AF is accurate and the image quality is unimpeachable. I have some 12x18 Fuji Crystal Archive prints from that camera that just floor me… But it's fundamentally a point and shoot. It gets less enjoyable to use as your approach becomes more deliberate and formal.
That said, after spending several months with a friend's 645 Wi, I'm thinking about buying my own. You get 16 exposures per roll. Focus is accurate. The lens is phenomenal. Exposure is bang-on reliable. The camera is light (plastic has its advantages) and it packs really really well. In fact I put it in my knapsack in a ThinkTank 50 lens bag. Perfect. And the results speak. Remember that this camera encourages you to shoot vertical format.
Dante Stella has a good page on these cameras.
http://www.dantestella.com/technical/ga645.html
The TLRs are great from a weight and reliability perspective (If it was good enough for Bourke-White it's good enough for us!). But you have to decide whether you like the square. For some it really works. For others, it's intolerable. These cameras have good resale value and are worth saving for. If you get one and it's not quite for you, just re-sell and the financial penalty is not severe. There are good tutorials on the web for how to assess a used Rollei's mechanical soundness.
Between the Mamiya 6 and 7, you want the 7. According to two friends who do professional camera repair, the extensible "bellows" of the 6 is a known mechanical Achilles' heel. The fact that the finish on the 7 is not durable works to your advantage. A lot of these cameras that look like crap are in fact mechanically perfect. And all the lenses are spectacular.
In terms of bang-for-buck, don't neglect to look at the Pentax 645. You can get amazing bargains on these cameras and the lenses are absolutely first rate. And the camera and lenses have AMAZING build quality given their prices. I defy anyone to actually use one of these cameras and not fall in love. Ken Rockwell does a good job of covering their strengths and (sometimes significant) weaknesses.
http://kenrockwell.com/pentax/645/645.htm
An alternative possibility that is well within your budget: Nikon FM with 28/2.8 AIS and either 50/1.8E or (for a more weight and a bit less speed), 55 micro. I prefer the rendering of the 50/1.8 E to any 50/1.4 Nikkor I've ever used, and it is a LOT lighter and it is CHEAP and it's only half a stop slower. One of those "bargain-of-the-century" lenses. Actually, I like the way it renders better than the v4/v5 Summicron, though it's not as viciously sharp.
The micro lenses have pretty much unsurpassed image quality. There are many versions, as discussed here:
http://photo.net/nikon-camera-forum/00Fgng. I'd get a 55/2.8 AIS because it has floating elements that allow it to perform well at close and distant focus, and it will reliably out-resolve even TMX or ACROS.
An FE with slow B&W film and a lightweight tripod, used at optimum apertures, is light, brutally reliable, extremely versatile, and not easy to top in terms of 35mm image quality. Honestly, at f/5.6-f/16, my Leica M gear is (under most field conditions) not meaningfully better — especially when compared to these especially good (and reasonably priced!) Nikkor lenses. And no parallax issues, either!