Right, The idea is to stop any rust, and do any patches in a temporary non damaging and easily removable fashion. Then once the truck is mechanically sound and you are knoledgeable and ready for the sheet metal repair/restoration, then you can start.
A good looking truck that won't run reliably or stop is not worth much when you need it, unless you put it in the front yard for a flower planter....
I too learned how to play with vehicles and engines by starting with a 52 Chevy 1 ton (when I was 16 or 17), I remember getting the tail lights working just before my birthday dinner, and going to get the state inspection sticker the next morning.
Then a few years go by and I end up driving the same truck around the country, 8,000 miles from coast to coast and back with some detours, all in 28 days. The truck did not look like much, all I ever did to the body was oil the rust spots, but mechanically it was near perfect, and we had only two problems on the trip, a broken fan (very unexpected), and a bad generator bearing (was rebuilt before the trip) nothing big, and both easily fixed in a few hours.
Tackle the mechanical part, then worry about the body.
Grigg