I think you'd be better off with an ex-Army truck. The 2.5 and 5 ton CUCV Gen 1's are being retired. They are extremely well built, surplus tires are cheap ($150/ea), Cat engines with off the shelf parts.... There is a whole community of folks out there.
Top end is 60 mph on them, but you can mod them for 70.... They are pretty high up though.. but you could build a truck camper in one without much trouble. I thought about it as a replacement for my Neoplan coach. I also thought about a Skoolie....
This vehicle is neat, but $4500 is just going to be the beginning. You could learn to weld, so the build out isn't that big of a deal. But doing it where it looks good is the harder part for exterior body work. When something mechanical breaks it's going to be thousands...... and hard to find. There just aren't many of these.
I'd advise you to get a set of blue prints for it before going any further. Drawings would work as well, should be readily available. A parts diagram will have the same info. Find out where the bulkheads and framing are at.
I'm doing a teardown on a Motorcoach (Bus that is already an RV) if you want to see what's involved. This one was overbuilt and has lots of crap that needs to come out.
I have loads more space to work with then I would with a skoolie. My Insurance is cheap.... $125/yr for liability, and most campgrounds don't give me grief because I'm rare and it looks okay.
You can find a vintage motorcoach for $5K that is liveable and just needs updating inside..... or you can take it apart like I did and re-do it.