You could buy that setup for about $15,500 out the door and it would be brand new (2007) with a new warranty in your name. The factory 2 year warranty is up. Even if it is in perfect condition, I would suggest paying no more than $12,500 The seller might have paid too much to begin with and is trying to recover his costs. He night still owe that much if he bought it with zero down and has not reduced the principal balance. Virtually any buyer without trying hard could get that package new for $15,500 and get JD financing in minutes
If you can pay cash, offer $12,500, be polite and leave your phone number. You will most likely get a call in a week or so. No sane person will steal it from under you at $15,000 Hauling an unneeded 790 to Arizona would not be high on the sellers list of things to do soon.
As I warned another potential buyer in a post a month ago, make sure you get a notarized bill of sale and solid proof that the seller is indeed the actual owner. Verify that there is no lien on it from a lender. His bill of sale AND invoice will confirm that. In some states, one of those documents might say cash and the debt might only show up on one document, not both.
Since there will not be a title like a car, make 100% sure he has the right to sell it. If he owes money on it, your check must be made out to the seller AND the lender. If that overpays his acct, he can get his original lender to refund the extra money in a couple weeks or so.