If it's in really good shape and has the battery changed, I would pay up to $250. Even then, I'd want a couple cables, an effect pedal (it does really well with a nice overdrive), or a stand. See if you can talk him down. In the future, try synthprices.blogspot.com
Programming is very basic and should be intuitive, but don't expect crazy live tweaking or anything like that. Personally, I love the P61 sound and could never remove the two I have from my rig. That joystick is a ton of fun, and chord memory can make some huge leads, and unlike the Juno 60, the oscillators can be tuned. As for the sound, to me it falls in between the Polysix and the Poly 800, so if you like that sound, you'll like this.
You can mod it to have MIDI, but from what I've seen, the mod kits are often as expensive as the synth itself! Regardless, I think it's a hidden gem that can be snapped up for cheap and as we see the older generations of analog polysynths become unobtainable or impractical to own, I think attention will turn to these. Check out what Jexus does with the P61 if you want an idea of what a properly programmed one does.
If you can't get him down, +1 for the Alpha Juno 2. Another DCO board that just screams, but in a whole different way.
32 MU modular system, Electribe (EA-1 and EMX), Microbrute, Nord Lead 2x, Blofeld, MS20, AN1x, Poly 61 (x2), JX8P, Split-8, DX7, Alpha Juno 2, microkorg, Conn Electric Band, TR-505, Ace Tone FR-3, Monotron Delay, Beatstep