i recently bought a 1966 mustang coupe. i bought it because i've always liked the way the old mustangs looked. mine looked like it was in pretty good shape just by looking and driving short distances. boy was that deceiving. the first design flaw that i became familiar with is the cowl vents clog up with leaves and stuff, then they rust out, then they leak into the car and your floorboard rusts out. both cowls and floorboard can be replaced but anyone who has tried to do the cowls will tell you how difficult it is to get to them. the second design issue was my car was originally a six cyl. but it had been replaced with a 302. nice motor but they didn't replace the radiator to accomodate a v8. you don't have that problem with a camaro or firebird. they use the same radiator for both 6 cyl. and v8. the mustang doesn't so you end up with a overheated motor. another design flaw that i encountered is that 6 cly. cars came with a 4 bolt pattern wheel and v8 cars came with 5 bolt pattern wheel. again, not so with gm. in my opinion the older mustang has several design flaws. they can be overcome with money. my 1966 mustang  now has a new radiator, new disc brakes, new cowls and floorboards, bodywork, interior, and wheels, all at a price. so, if you are looking for an old mustang and your not a ford mechanic. beware, it could be very costly for you. with that said, mustangs are very fun to drive so weigh what you want with what you can afford to do to it. i hope this will help some of you that are in the same boat that i was in. good luck!