Rust and structural rust. A little rust spot here or there an a 40'year Old car is to be expected and can be repaired. Structural rust should be avoided. Shipping a car from the west coast to the east coast may cost a couple thousand. Repairing deep structural rust may cost two to three times that.
Paying less in the beginning may cost you way more in the end.
Note: I'm on vacation an d drinking cocktails so what I just posted may be total bullshit.
