If my memory serves me right, the major difference between the A and C is the engines. Their may be several avionics and other "black box" upgrades inside the airframe but they are invisable to the outside.
Monogram made the A and C kits out of the same tooling in 1/48th scale, I think they just added the differnt exhaust nozles for the C. It's a great kit that can be found for less than $15. The only thing I would replace is the Aces II ejection seat. If you do some searching around, someone may have the old A afterburner cans.
Minicraft Academy made an excellent 1/72 scale F-15A. See if that's still available.
The E is very different, it is a 2-seater and has the FAST packs under the wing.
"I love modeling- it keeps me in the cool, dark, and damp basement where I belong"
Current Projects:
1/48th Hasegawa F-14D- 25%
1/48th Tamiya Spitfire- 25%