You can always get a full body paint job in Mexico for $4000 or so. And then, you live inside so it's much more important that you like the insides then the outsides anyway.
These days, with so many RV parks putting in 'pull though' sites, I wouldn't worry about length that much. You will pay a bit more than someone with a 20 footer at older parks, but don't worry about that too much.
As far as the mechanicals, you can hire an expert to check it for you. Cost for a 40 foot Diesel would likely be in the $400-$600 range. Be sure they have the certification. But it's worth it if they approve the engine and tranni. Huge expense if those are in bad shape.
Of course if you're a wrencher, check for water in oil and oil in water everywhere. Everything else is simple and relatively inexpensive in my book. After 17 years of full timing, I keep an over $10K emergency repair fund. 'Cus you never know. Don't bother with Service Contracts (often called extended warranties but they aren't), few ever get their money back in claims. Just open a repair bank account and self insure. Back when I was accumulating my repair fund, I was putting $175/month in it, and that barely exceeded what I was paying for upkeep and maintenance. Whoa! But it's true. Still paying $212/month for upkeep. This is not an inexpensive hobby. But to me, it's a lifestyle.