Check it out as well as you'd look over a used car. If they haven't been maintained well, you're looking at a whiole pile of repairs. I have mine stripped most of the way down now so I can start replacing everything that's busted, rusted, etc ... and was shocked to find how many mickey mouse repairs had been done. Gonna have to grind (or torch) out some pretty bad welds and try to do a better job than "he" did.
My guess would be the first thing you'll want to check is the condition of the gearbox and the connections from the PTO to the side shaft first ... they'll give you a pretty good idea of how well it's been maintained.
Was just going to post this and then I remembered ... I just saw an almost new spreader here in mid-Michigan for $950 ... but it was ground-driven .. don't remember the make though.