Go horizontal, and hang the 18" piece with the cut edge down toward the floor. That will give you a factory edge to factory edge joint when you hang a full piece above it. You won't have to tape/mud any **** joints that way.
+1 for the bush hog. It sounds like a no-brainer to me. Cheaper, less maintenance, and faster. What else do you need? A tractor with a bush hog will go more places and do it way faster than any of your other options, IMHO. It also gives you flexibility when you don't have time to mow the...
Good score on the DeWalt!
I'm a little late to help you out, but just FYI for anyone else considering a 1/4" hex drive cordless impact. In the past 2 years I've had a Ridgid 14.4v 1/4" hex drive(stolen) and currently have a Bosch 18v 1/4" impact. Both were beyond my expectations for carpentry...