All up to you now!
Do your homework, get hold of some Mustang books, plenty out there; quiz your uncle about his car, compare his '67 to others, that's awesome that a member of your family has already got a Mustang; go to car shows & study them up close, talk to their owners, car people love to yak about their pride & joy; immerse yourself so you are very informed.
Take your time, be ready to pounce on a good one, avoid the roughies with rust they will cost you just as much maybe more in the long run. And, make sure you finally buy with head not heart.
Buy well & you will enjoy a very satisfying hobby.