There are lots of very affordable options, but I would agree with others and suggest the Mamiya 645. I recently sold my Pro TL and it was a great little system. There are tons of lens options and they are very affordable. I really liked the 35mm! Also this system has the fastest MF lens of any, the 80/1.9 and a great macro system with the spacer that gets you to 1:1. Also the Pro and Pro TL have a pretty modern ergonomic feel and function to them, with built in and very reliable meters, aperture priority mode, and the motor-drive. In my experience, at least here in Japan, the Mamiya 645 system is cheaper than Bronica, Pentax or Contax and more widely available than all but the Pentax.
The others are good options too, so my comments are only a recommendation.