The performance improved heads, cams, and intake were first offered in the 1999 Mustang GTsAs Shelba toast says it all depends on what you want, I have done two, one with a 89 5.0 and am currently finishing up a 4.6 SOHC. THe 89 Donor car has 10K miles on it. I drove the 4.6 car about 400 miles then tore it apart for paint etc.
Both were relatively easy for me, I was fortunate to get a 17,000 mile donor for the 4.6. I used everyting from it I could. I paid $3500 for it and it was well worth the money, it was wrecked hard.. but just right for me. , clipped in the right front and rolled. I had to replace the right front spindel and one rear axle, otherwise I was able to use everything including the radiator.
The 5.0 car was a rebuilt high MIleage car that I paid $2000 for. It also worked out well although I did have to replace some of the items, I replace axles, drums and rotors to convert to 5 lug, new radiator, swapped the tranny for a 5 speed, changed rear gears etc, etc.
Donor or non donor also depends on registration and inspection requirements in your state. MAke sure you understand those requirements before buying anythign or you coudl get caught witha whole bunch of money invested ina car you can't use.
Somebnody will chime ina ndsuggest a lincoln MKVIII many are doing it, but it seems like a lot of running around to me, but you do get the more desireable motor.
You'll want to stay away from the 94,95 mustangs as they are not as desireable, they have the 302 but front pulley system and accessories won;t work, they also have the less desirable wider rear axle.
96-98? 4.6 have the older heads before the performance improved heads were introuduced and you would wantto stay away from those as well. I am not 100% certain when the PI heads came out but someone will correct me.
Again as Shelba toast suggest ask away we will be more than happy to help spend your money.