I'm not sure what year Mustang you have, but here's what I have ended up doing with my 66 Coupe.
So far I have bought 2 gal of Epoxy and activator 1 waterborne (700) and one solvent (710) W&G remover. And one 895-1 reducer to have around when I seal the primer (if ever lol). If I don't use it all I'm not that worried. I am sure I can find stuff to paint, plus I don't mind spending extra to support the absolutely awesome service and guidance I've received from SPI.
I didn't buy it all at once, initially I just bought the cleaners and 1 gal epoxy + activator. But by the time I had practiced spraying, made several mistakes, messed with gun settings, etc, etc I went ahead and bought more since, like you, I plan on putting the whole car in Epoxy. Including items like the rear end housing, suspension pieces, trans case, etc.
If you're a noob like me and your budget allows I'd go for twice what you think you need
Here's my forum posts, all the guys have given me good advice, and you have the bonus to learn from my messups!
http://www.spiuserforum.com/showthread.php?1761-1966-Mustang-Coupe-a-DIY-story