Depends on what you want to do I suppose. If you just want to monkey about with old junk (uh... good old junk that is. Junk we all love) then pick up a used M1 or T or an O1W. Whatever you can pick up really since the stuff is becoming somewhat of a rarity. Even if you manage to get one in a good condition any potential repairs will run you up more than the thing is actually worth.

IMHO you'd be much better off investing in a nice audio card, some sequencer/ host program (Like Sonar Home or Fl or Cubase) and a bunch of software synths. It might cost more than the M1 but by the end of the day you'll be investing in a really good setup and not just one half-dead old workstation. As reliable as some old synths might be they weren't meant to last forever. ...But then again, nothing lasts forever, right?

cheers,