For what its worth, everyone seems to be afraid of rust but seems not to be bothered as much by mechanical needs.
My car was ready for the crusher when I got it; a non-sense restoration undertaken merely because it was a family legacy.
Based upon my best estimates, ignoring some unnecessary baubles such as the all leather interior, new windshield, etc. there is as much in mechanical restoration as there was in body restoration. Further to that, the mechanical restoration continued long past when the the body was done. Tracking down and repairing myriad little things such as broken wires, etc. can be enormously time consuming and also expensive. Sometimes you simply have to pull some parts swaps to get to the root of the problem.
Keep in mind that the body restoration on my car was quite extensive and included a new nose, tail, hood, and various other sheet metal sundries. Yes it was good news for K&K.
That does NOT mean go out and find the car with rust and buy it. It does mean that the entire car must be taken into account, and body work is not necessarily more difficult nor expensive then mechanical work. In fact, one could argue that any reasonably competent body shop can do the proper work with a little guidance. Mechanics? Well, we seem to be all searching for them nearby.
If your "target car" burns oil, has a questionable fuel injection pump, perhaps some rust in the fuel tank, a slipping transmission, and a leaky rear end (hey--throw in a seized caliper, too) but has a rust free California body...add up the mechanical cost of making it all right. Then compare it to a car that needs some floor work, maybe some rust repair in the usual spots--but otherwise runs great--and you might be surprised. They'll be closer in cost to make right then you might imagine; which way is anyone's guess and depends on the car of course. Conventional wisdom might tell you that the body work is more expensive. I'm here to tell you it isn't always the case, and I have a pile of receipts that justify that assessment.
When an engine rebuild is about $6K, a measurable portion of that a head job, a fuel injection pump rebuild is about $1K, a fuel tank $1K, a rebuilt tranny several thousand dollars, you can easily get the idea that mechanical work isn't cheap. That theoretical car mentioned above could easily top $10K in parts alone; a lot more if you throw in professional labor. If you are doing it yourself you still have the parts prices. Mechanics earn as much as body men.
Just look at the entire package and take out your calculator.