Find an old Juno 106, its anolog it sounds great and has MIDI, however if your just looking for a controller maybe you could find a used yamaha cs2x for under 500$. thats a pretty good synth and would be a great controller for Computer based synths and samplers.
I couldnt have said it better! The Juno 106 is a absolute classic! The sounds that come out of that thing are like no others. The cs2x is a great synth though too.
I know "Musicians Friend" had a couple on limited clearance.
Get a Juno, its the best way to go. Try ebay they always have em.