The following answers are from my experience in building a 24 foot freightliner. there are many more overall experienced cats in this forum.
1)vehicles rented by U-haul are usually wasted by the time of sell-off.
the space under the box can be extremely useful. besides the obvious generator and air conditioning space, there is easy access to running exhaust, electrical, plumbing, etc.
make sure you drive each vehicle. I ended up buying a freightliner because the ride was so much smoother than the internationals. the internationals were much quieter than the freightliners though.
2)The FRP looked like a great idea, but the aluminum made it easier to insulate with extruded foam between the studs. I then put plywood over the studs (on the inside) and glued plastic sheeting on top for a finished look.
3)I removed the seats and rear window from the cab, then took the drill, sawzall, and die grinder to the space. Use much patience. you can get an accordion seal from uni-grip.
4)believe it or not, $20k and a massive amount of sweat equity. I am still doing some finishing to the inside, but after two trips I am pleased. It doesn't ride like my Silver Eagle did, but I have lots hauling space, a dock high loading gate, and plenty of clearance for when I am at construction sites.
Doc Weaver '98 Freightliner FL60 pseudo toy hauler