1.) What do you want to do with the car? - Daily driver/street cruiser, Street/strip or strip only. Each one has its own combination of parts that work best.
2.) Once you have decided what you want to do with the car, how big is your budget?
3.) Plan on replacing the front spindles to accommodate larger brakes, make your choice of 4 lug (87-93 5.0 Mustang parts) or 5 lug (94-95 Mustang parts)
4.) Plan on replacing the rear axle with an 8.8" unit and some gears - at least 3.55:1. Again, what you choose is a function of what you want to do with the car. If you can find a 87-88 Tbird Turbo Coupe rear axle you get 3:55 or 3.73 gears in an 8.8: axle with disc brakes, It's a very nice setup when you use the right combination of parts.
5.) The stock suspension is probably pretty worn out, so plan on replacing/upgrading the suspension parts as well. You'll need new control arms or bushings on the rear axles, new shocks all around, some full length subframe connectors (weld in only type).