I agree with the casters. The other classes are really just specializations from the fighter/rogue/sorcerer, so personally, I would say you have to be at least level 7 before taking any other class other then fighter/rogue/sorcerer. Maybe draw up a quick and dirty enchanting system, or simply say that certain weapons you pick up can only have so many enchantments on them, so max +3 (or three glyph slots) for on weapon or armor. And base the number of enchantments you can have on a certain weapon or armor based on the material its made out of.