You would be hard pressed to get more for your money than a Mamiya RB67 - a truly professional system that you can grow with, that I have seen go for as little as $200 + with one lens, one back, and a basic waist level finder. It is a little bit big though, but I think the features more than offset that - especially the close focusing ability with the bellows and the top-notch glass.
There are also the Pentax 6x7 cameras - aside from the little quirks that they have, they are just about as seamless a transition to MF from 35mm as you can ask for - they look, feel and work exactly like a 35mm SLR... on steoroids.
Another budget idea would be a Kiev 88 - its essentially a Russian Hassy knock off (I believe there is even a high degree of parts interchangeability), and from what I understand, the newer ones that are officially imported come with warranties and generally seem to enjoy a pretty good reputation. They are a fair bit smaller than a RB67, and also quite flexible as they are a full system with various lenses, finders, etc.
Personally, the RB 67 is my choice - but I would strongly suggest getting your hands on one - literally - just to see if its not to bulky for you. The only draw back I can see is that the 6x7 format will give you 10 frames from a roll of 120 - it is a bit of an adjustment for someone used to 36 frames on 135. But, then again, so is 12 frames of 6x6.
Best of luck - its kind of hard to go wrong, there is a lot of quality stuff at very decent prices - all of it blowing 35mm out of the water in so many ways (keep in mind, I am an avid 35mm user, so this is no knock on the format, there is simply... only so much to play with!).
Peter.