Dude, IMHO, there's no hmmmm about it. I know it's easy to spend other people's money, so be very careful about the rest of the machine, but sure seems you'll end up with a very nice machine for less than 20k. And, I'll say, I've never changed an engine on a combine, but I wouldn't be scared to try, I can visualize it as being fairly straight forward.
Good luck to you, you have an interesting decision on your hands.