http://www.prepal.com/data/ARP.htm
I paid about $700 for my Odyssey II used when I got it back in 1980. Things to look for: check all the pots and sliders as Rozzer said, and definitely check every key...my Odyssey was notoriously sensitive to how hard I would hit the keys: there are two wires under each key that connect to two buss wires (one for each oscillator, I believe) running the length of the keyboard when the key is pressed down, and sometimes if I hit the keyboard too hard one of those wires would get stuck under the buss and freak the thing out (sudden WHOOOP! when you reach the note, and then all the remaining keys to the right would be stuck on that one note...quite embarressing in live situations
). Was easy enough to fix (open it up, slide the wire our from under the buss wire with a toothpick), but something to keep an eye out for. 
One thing you might also want to keep an eye out for if you do get the Odyssey is a used Roland MPU-101 MIDI->Cv Interface, which allows you to control it via MIDI. Another perk I discovered with the MPU-101 was that the Odyssey's tuning would become rock-solid when connected to it and almost never drift. So if you ever see one of these on eBay or something, grab it!
(not to mention it can be really handy for other things as well: these days I've been using the MPU-101 to sync up my Moog Moogerfooger MuRF pedal to SONAR)