Now that is an interesting creative calculation.  Unfortunately I do not see is context in reality.  Reality is that the interest is not for the life of the tractor and the added interest of that $20,000 actually goes down each year.  As far as an appreciating asset, that is a worthwhile consideration, but a little over simplified.  There is a formula that takes all this into consideration that is plug and play, I just don't have it at the touch of a button.  It quickly lets you play scenarios that will let you determine whether the magic difference is even, 5k, 10k or whatever.

Used to do those calculations for vehicle purchases and large projects all the time.  Sometimes to the point of wondering "what's the use".  Gosh, I hated doing that.