I'll still say, although you appear to be about 100% set in what you want and will not waiver, if your building something for the long haul, it doesn't make much sense to start with a 15-20 year old trailer. Especially in the rust belt.
Don't look at it as a % of new vs used cost look at total $ out the door.
$3k (if you can find it, or maybe you'll sit on the couch indefinitely until you find one) vs $5k for a trailer isn't much.
You'll start off with new bearings, brakes, tires, Jack, breakaway system, wiring, etc....a new trailer!
If you have to work on all that stuff or replace, you'll eat up that $2k mostly in money, or more in time too and still have an old trailer.
The 12 valve thing I get. Significant difference between that and newer trucks and the simplicity is a bonus. Even with the 12V though it's a point of diminishing returns.
But trailers are trailers. They haven't changed in 30 years. Certainly not for the worse. And your one experience with a leaky roof doesn't mean anything. If it leaks fix it, if it still leaks, you don't know how to fix a roof.
