I can't help with most of your questions, but a couple. When I rented a 19G in Las Vegas for a seven day drive to Zion, I never once saw the solar charging light on. There was a lot of sun at the time. You can find the price of new engine and transmission easily. There are several businesses that do just that.
My opinion is buying a used rental is buying a higher mileage newer model compared to a comparable older Class C with lower mileage. Because they are bulletproof, some systems, TV antenna for example, are not there or a little more basic. One of the first things I would do is make it mechanically reliable, tires, etc and put a nice efficient solar system on board, That allows cutting the cord on many crowded campgrounds and allowing you to dry camp. For me, that is a good thing. Have fun!
