If that tractor is in good shape, the $20,000 is a good price, but nothing wrong with seeing if you can get it for $18,000. If you don't try, you'll never know.
Any warranty left on the whole tractor (two years from original purchase)? You would have another year on the powertrain (3 years). That's worth a lot when you consider what could go wrong in the next 1-1/2 years. Without that warranty, I would definitely ask for $2000 off the asking price.
Is this a dealer or an individual? If it's a dealer, the warranty should be very easy to arrange.