Rust is one of those things where for every bit you see, there's usually
twice as much more hiding. Personally I wouldn't even try to fix a rusty car
unless it was a rare classic, but then rust is relatively rare here and
completely rust-free 80s Volvos are not hard to find. The best option
depends on the amount of rust damage, surface rust can be sanded off and
filled, rust holes need to be cut out with new metal welded in. For paint,
you get what you pay for. You can improve a Maaco job significantly by doing
all the tedious prep work yourself, but you're still looking at a couple
thousand dollars for a decent paint job that's anywhere near what the car
originally came with.