Looking For Advice On What To Buy

99-04 GT's use a 4.6l with the 2v SOHC cylinder head and are rated at 260hp
05-10 GT's use a 4.6l with the 3v SOHC cylinder head and are rated at 300-315hp
11-14 GT's use a 5.0l with the 4v DOHC cylinder head and are rated at 412hp

Your simply not going to be able to make the same power out of the 99-04 GT's as cheaply as you can with the newer models but for about the same price you can get a 99-04 Cobra witch uses a supercharged 4.6l with the 4v DOHC cylinder head has IRS and are rated at 320hp for the 99-01 and 390hp for the 03-04

Prices for parts from what little iv looked into it seem to be about the same for the 99-10 GT's

Convertible are heaver do to the motorized top and stiffer subframe witch dose make them a bit slower but you gain a convertible

I hope this helps a little I more or less just coped stats I found online