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 :)