Yeah I wouldn't bother, $3600 he is dreaming. For that money I'd be expecting something that has a nice body and maybe an 18RGU twin cam.
Rust under the door hinges is pretty bad. Last thing you need is your doors falling off!
There will always be more rust sitting under the paint around the visible holes. Most places I removed rust from around my RA23 had a fair bit of rust around the holes that you couldn't see without grinding off the paint.
They are a pain to repair, the metal is quite thin and burns through easily when welding if you're not careful. Like I said before you are better with a good body first if you can find one. There are plenty of good 18R's floating around so finding an engine for one isn't going to be a problem
Best thing to do is take your time. If you rush into it too quickly you will regret it. Good Luck!