Oneshot, man It's none of my business other than you posted it. The interest rate he wants is 2 1/2 times higher than it should be as well as the BS closing costs. If you wanted the land, I'd offer him the 18K and offer to pay closing costs, but go thru a bank instead. He'll get his money NOW and you'll save a ton of money and be the clear owner sooner.
You could likely find 4% from a bank, finance for 10 years. If you put the same $1000 dollars down, Your note would be $173. You would pay $3700 (or there abouts) in interest over the life of the loan.
HIs way has you paying $164 a month, but because of the higher rate and the 20 year term you would be on the hook for > $22,000 in interest.
I'm all about being happy where you are though. Just food for thought.