Mamiya 645
I use Mamiya 645 and must say that it is incredibly cheap to invest in. A mamiya m645 1000S is a very capable body with 1/1000s top shutter speed, mirror lock-up and depth of field preview built-in.
These cameras are fairly old but are very solidly built and can be found very cheaply (sub £200 for a complete kit including finder and standard 80mm f2.8 lens)
The m645J lacks the features from above with only 1/500s top shutter-speed, but costs a fair bit less. I bought an m645J body with a 120 film insert for £35 a few weeks ago, to add to my 1000S. Accessories are what put the cost up. But a basic system includes the Body, a 120 film insert/Back, a 70mm/80mm/110mm f2.8 lens, and either a prism finder or a waist level finder. If the prism finder lacks TTL metering, you will need to use an external light meter to determine the appropriate exposures. The metered Prisms will give you aperture priority only, where you set the aperture on the lens and the camera will automatically set the shutter speed.
Bronica medium format equipment can also be found quite cheaply too, but i have not got much experience with them, though i do know that the lenses are well regarded. (The ETR Series is another 645 format system) The pentax system although very good can be slightly more costly than the others.
All of the above medium format systems are manual focus only, there are AF versions of the mamiya and the pentax but they cost a hell of a lot more, since those bodies can have digital backs fitted to them.