I got the Mamiya M645 1000s working last night. I thought it may have been broken because the winder was essentially rigid and the curtain inside the body was half up. I ended up being able to (with some effort and care) rotate the winder and cock the shutter, and the curtain raised properly. I then fired the shutter and the winder was then no longer stiff. It is a little tough to cock the shutter after advancing but otherwise it seems happy.
I bought a battery ($16 from Rainer's! I'm sure I can get a better deal), and the prism seems to work. I suspect its metering may be a bit off though. Not that it matters, I'm happy to do manual exposure.
Ordered a grip and film magazine from KEH.com last week, and they should arrive soon. Then I'll be able to test it properly :)