There is quite a difference in the mustang from 94 to 99.
The 94/95 GT has the 302, which you get make huge power for relatively cheap (plus it's the best looking car
)
The 96-98 GT had the first generation 4.6L, I think these cars respond well to boost (and look pretty sharp too
)
The 99 GT has a lot right from stock. These cars also respond well to boost. Like
StoplightWarrior said, you'll be faster than most cars with a 99 GT with the mods he listed. Just keep in mind that it will cost a lot more for parts if you want to go further.
They are all really nice cars, pick the one you like and can afford
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1995 Mustang GT