You don’t want to have to contend with a literal rust bucket with perforated body panels or floorboards, so look for dry vehicles that have had no exposure to winter road salt, which generally means the southwest.
As far as type of vehicle, there are still a lot of earlier Mustangs around, which tends to depress the price, I favor the fastback model personally.
Another possibility would be performance models of midsize two door sedans, but even some of these have gotten pretty pricey.
The El Camino and Ranchero haven’t gone through the roof other than top of the line high performance models. The El Camino looked pretty good in its last iteration that was produced into the mid-late 80’s. You could probably pick up one of those for a reasonable price and de-smog it if you aren’t in CA.