If I could buy old manure spreaders for 20 bucks apiece, I'd do it all day. They're a hot item with the part-timers and horse folks- They buy a tractor and bush hog, and the next purchase is a manure spreader.
From the description, I think you did just fine at $600, but it wasn't over priced.
When I was a feckless youth, I was building a hot rod and bought a Chev 283 V8 at a local wrecking yard for $125. Stopped at a store on the way home for a Coke and a Moon Pie, and a guy offered me $200 for it off the back of my truck. Didn't take it, but in retrospect, should have done the deal and gone back and got a 327!