I have a pretty extensive Bronica S2A system, and I like it. The S2A is more rugged than the S2, but if you've found an S2 in good condition, and you don't abuse it (be particularly careful not to overwind the shutter), it should work fine, and that's a reasonable price. The Nikkor lenses are great, and you can build a very versatile setup for not too much money.

A common problem with S-series Bronicas is that the foam underneath the groundglass frame deteriorates, causing focus problems. It is easy to fix, but you should check it before purchasing the camera, and if it requires replacement, have the shop attend to it before you buy it.

For information on all things classic Bronica, check out this site:

http://medfmt.8k.com/bronica.html

The Mamiya 7II is a great camera with some of the best medium format lenses available today, but it's a rangefinder, rather than an SLR, so it's a very different kind of choice.