There would be very little difference between the balers other than the pickup. The JD 328 Should have a wider pickup on it. The price is not terribly out of line IF it has as little wear as you say but it is getting close to twenty years old.
The trouble is there are new balers out there at bargain prices. Koenig at Greenville Ohio has a new 2016 baler for $18,900. I bet that $17500 would buy it. You would get warranty on that baler. There is a new one down in Texas for $14,999. Age does matter too.
One of the worse purchases I ever made was a car that only had 10,000 miles on it but had very little use for ten years. Every single seal on that car leaked because it had set so long. In the first year: alternator, water pump, rear end seals, transmission rear seal, and crankshaft seal all had to be replaced. So I wonder what kind of startup issues you would have with the neighbor's baler after it has set for so long.