rust!!!!!!
back and front windscreen, 1/4 panels and around the subframe.
if you can, try to buy a west coast car not a east coast car (east coast has snow which means salt)
water + salt + car = rust.
also be very careful of ''missing parts'', while the mustang is very popular and most parts are made for most models, not everything is made and can be quite hard to get ahold of.
anyways good luck.