What should I know about a 1968 Mustang?
You should know that the car is 43 years old, and that it will cost you an arm and a leg for insurance.
Assuming that it has been well maintained throughout its lifetime, (probably not!) maintenance shouldn't be much of a problem, but go buy yourself a good service manual for the car and spend some time reading through it BEFORE the car breaks down.
Also, before you buy it, check underneath for rust. Pull back the carpets and see what condition the floorboards are in. Rocker panels can also have lots of rust, and are expensive to repair. I have a good local mechanic that I know and trust, and I always have him inspect any older used car I am considering before I make a final offer to the seller.
Good Luck, and don't say I didn't warn you!