Keep in mind, the Fox body mustangs don't pull a lot of money unless they are very low mileage or they are a cobra, so i'm guess hes probably going to want a car that doesn't cost a lot. For that reason I would rule out the newer body styles such as the 99 and 01. Plus, if your going to dish out 10-15k for an 99 or 01 you might as well just save up a little longer and get BEAST mentioned previously (03-04 Cobra).
That being said a 96- 98 would probably be your cheapest route. Horsepower between the 96-98 and 99-01 is pretty close, the only thing you're lacking is the IRS. Depending on what you want the car for the solid rear axle is going to be stronger, however it will also not handle as well as the IRS.
By the way, if you can wait and save up for an 03 04 cobra I think you'll be even happier. I've been looking for one now for about 3 months and the prices are getting pretty low. Recently I saw one sell for 11,500 with a clear title and about 100k miles. Despite being high mileage I still think that is an excellent deal. But for the majority which have lower then 50k miles the prices are generally just 20k. From what I've read and seen these cars perform just as well as the 07-09 GT500s. | 
