Fortress is a game about limits, you are limited to your weapons and you can't change them. That's the whole idea of the game. The Buying system will NOT work in Fortress! Should I write that on a nail and hammer it into your thick head??? Fortress shouldn't be used with any mutators at all, reguardless of where they are from!
On the other hand, we need a Buying system for Deathmatch, CTF, and Gauntlet games! That I can say with absolute certainty! The ablity to choise the weapons you want for the situtions you face has been very important to many mods, and it's been a great oversight that U4E hasn't had that option yet.