Those are pretty common bolt on mods for the 99-04 GT's. You can add all of those engine mods and don't require a tune, so you're fine there. A tune would make them more effective, but it's not necessary.
The eibach springs should be fine...a 1" drop is hardly noticeable...usually the eibach's (pro kit) lower the car 1.5", so I'd check into that. A common rule for our mustangs (v6 and v8) is that if you lower the car 1.5" or more, caster camber (CC) plates are necessary for proper alignment to prevent uneven tire wear. I'd check for aftermarket CC plates on the strut towers...here's a pic:
Aftermarket shocks/struts are always a good idea since the spring rate is different from stock...so the stock struts/shocks are going to wear out quicker. You don't have to do them immediately, but I'd consider them in the near future. Bilstein or Tokico are both good brands.
You can have subframe connectors installed. They will stiffen up the chassis significantly, elminating chassis flex and help with launches and keep the car solid in the the turns. Weld in are the best bet, forget the ones that bolt in. You can do a strut tower brace as well, it will help with flex in the front of the car. That brace will eliminate the the strut towers from twisting. They run about $100-150.