Well, here we go. This is the situation when you're thinking of buying a newer used tractor. Often, the new price is not that much more, and financing can be 0% or low percent rates, making the new tractor a more attractive deal.
For that model, I believe you still would have the balance of the 2/1 year warranty so that's not an issue.
The best way to price it is to price a new DK45 in your area and take off a couple of thousand for depreciation. 40 hours is really nothing, not even broken in (50 hours), but it is used.
I would say that if you could get it for around $18,500 that would be fair to you, but maybe not so fair to the seller.
One thing you must be certain of is that the tractor is paid for and not financed somewhere. I'm not sure how you go about this, but another member may be able to enlighten us. I believe we've had threads about this in the past. There is a UCC (?) filed on financed tractors that would let you know it's not paid for. I do remember a thread where the poster talked of buying a tractor that had not been paid for, and lost the tractor and money he had paid the unscrupulous former owner.
The price I gave is not based on a good knowledge of recent prices as they have gone up since I knew what they are, and regional differences can make significant change in pricing. Check out the new ones first, and let us know how it goes.
John