Regarding the Moog Opus 3:
I own an Opus 3 and just something to watch out for before buying one. The Opus 3 was manufactured with a thin sheet of foam that rests just over the circuit board. Since this beast was produced over 20 years ago and depending on where the owner stored the keyboard, the foam has a tendency to turn into a black sticky goo which can be a major pain to remove from the circuit board as well as creating some sticky pots. Aside from that, I've run across a few non working units that only needed a new fuse to get them going again, which is pretty easy to do yourself.
It's a great machine, heavy as hell though.