My original Pentax 645 broke in an unusual way -- the film advance stuck and would simply wind the film onto the pick-up reel. I ended up selling it for parts after my local repair shop failed to make any head way after 6 weeks.
If I remember correctly, the original Pentax 645 uses a CR1220 battery for buffer memory, and the battery has a lifetime of about 5 years. It's cheap to replace, but you need a spanner wrench in order to do so.
I now have a Pentax 645n, which I use with both manual focus (45mm, 135mm LS, 200mm) and autofocus lenses (75mm, 120mm Macro, 45-85mm f/4.5, 80-160mm f/4.5). I even use it with a 300mm f/4 from the Pentax 67 system.