If you can afford it go diesel pusher. We're on our third and like the quiet ride and suspension. The engine brake will help you on your downhill journeys. The engine will outlive the MH :-)
Tow 4 down and skip the dolly. The dolly is a real PIA at campgrounds particularly if you have a back-in site. You'll have to move it by hand and it was pretty heavy for this 60 yr old bod. Our decision to go 4-down permanently came about when the DW accidently drove the Prius off the front end of the dolly. The wrecker operator was good and didn't laugh too much.
I'm not convinced a residential refer is the way to go. I think they were designed to be in a non-moving house. I just don't know if they will withstand the rigors (shakes, bumps, potholes, etc) of the road. Kind of reminds me of basement AC which you don't see any more.
If you're going to be in it for 6 months+ at a time get a 38-40ft. You'll appreciate the storage and room. I wouldn't get anything bigger than that as some campgrounds can't accommodate 42 ft+. We took a 5 month trip from NY to Alaska and back and the room was nice.
If you're buying used I'd recommend going through a dealer. They do have a reputation to maintain and you'll have recourse to the BBB and other things should you get a lemon. We've stayed with dealers on our 3 purchases and have been pleased. Might pay more but you'll have some reassurance. Just my opinion.