I don´t think this era mustangs rusts more than the others... I´ve had 66s, 70s and now this 72 and this is BY FAR the best built of all classic mustang i´ve had...
Rust always depends on several issues as the guys have been saying in here... As for the body design, in 71-73 cowl is the most common rusted area but fixing it in not that complicated... I prefer 100 times to fix a 71 73 cowl that fixing the same in a 66...
Our era mustang have a fantastic strength on the outer skins as doors, fenders and rear quarters dont loose their original alignment that easy....
Shock towers with the welded bridge down there is a gigantic improovement in the car structure and really make a difference in the feeling / safety when you´re behind the wheel...
I think what you have to look for is a non accidented car, with its original body panels and no signs of bad jobs done on it in the past... If you accomplish that, then you dont have to worry for the cons this body typed mustangs have, because for sure, they are the best you can get on classic stangs...