For the budget conscious medium format SLR shooter I recommend these cameras:
For 645 the Bronica ETR series which you are looking at. My first medium format camera was an ETRsi. Nice modular camera at a budget price today on the used market.
For 6x6 the Bronica SQ series. If you like square this is a great way to go. If you are going to crop all your images to rectangles then save some money and buy the ETR series.
For 6x7 look at the Mamiya RB67 and the Pentax 6x7. These cameras have fine optics and produce nice large negatives. The downside is that they are bigger and heavier which may or may not matter to you.
I have also owned Bronica EC's and an S2a. They are fine cameras with some excellent optics at a budget price but like someone mentioned earlier they are getting pretty old.