What are the common problems does a ford mustang gt have?

I'm tryng to buy a ford mustang gt 2002 and i want to know if those cars are bad or not and if the parts are expensive and if they waste to much gas

Common problems? Ricers and camaros constantly trying to race you, spending time at race tracks, etc

Seriously, the 99-04 Mustang GTs are pretty darn reliable. The drivetrain is fine and the engine will easily last 300k miles without blinking. Most parts are fairly cheap, since the SOHC 4.6 was used in literally millions of vehicles from the Mustang, to the F150, and SUVs like the Explorer and Expedition. If something breaks in the valvetrain, though, it can get expensive quick, but that basically never happens.

As for wasting gas, that would depend on how you drive. Drive conservatively and you can get high 20s/low 30s on the highway and low 20s city. If that's wasting gas for you, then there you go.

I have a 98 cobra, more power than the 99-04 GT, modified, and get about 21-22mpg city and 28-29mpg highway.

Great cars, very reliabile I got a 99 cobra. Common problems is rust in the trunk and rear wheel wells. But very solid and strong motors. Parts are very cheap for a gt. (For a cobra though omg price on every part is quadrupled the guy above must know with the cobra)