Having done both repairs and restorations on the earlier Mustangs. The best advice I can give you is to buy the best restored one you can find in your budget range. Even if it is a low optioned 6 cylinder coupe.
I would even budget the money to have it checked by a knowledgeable mechanic. Not only for the mechanics ,but for hidden damage and rust or rust repair.
Properly repaired rust is not a cause for alarm because the early Mustang's are going to have some . Even southern cars can have floor pans replaced, the biggest concern is the torque boxes and the cowl area.
I did a 69 Boss 302 for a guy who got the car from south Ga. It had been wrecked and sat for a long time. The whole area up under right front fender and the defroster vent had to be replaced . The 65-66 Mustang's have been around for over 50 years and have been passed around from who knows how many owners.
You may find one that's been in one family forever and has been well cared for, or you may find one that has been 2 cars put together from a bad wreck.
I bought a 69 Camaro once that looked and drove great. You could not tell it had been cut in two and pieced back together. I found out when I went to replace the carpet and found the splice.
Just be careful and don't jump on the first one you see. Take your time and make sure the paper work is correct. People will put lipstick on a pig and try to sell you anything. Don't believe half of what the seller tells you unless they can prove it and then still be skeptical.
I sold this Mustang for 18,000 and it was a show winner. After I got done rewiring it , redoing the interior.