For $400 there's a ton of great s/h modules available. You could probably buy two for that amount. Yamaha TG500, 64-voice polyphony (and some people are giving these away). There are several great Proteus modules from the 90's. Don't forget Roland's JV-series. JV-880 and maybe even the 1080 is within that reach. Depends on the amount of expansion cards stuck inside. Emu made a bunch of modules aimed at the dance market which I've seen really cheap s/h.

-tek