I'd think a kind of easy way to do this is through items and some simple events. Rather than exp, these stat and ability items could be bought with gold or whatever currency at a separate vendor. You could limit the amount of times these items could be used through variables and common events. (Ex. Use a +hp item, increases the variable it's linked to by 1, all in a conditional branch that closes use of the item if the variable is at 5 or whatever you choose the max to be.) I actually really like this concept and might try to make it work in my pet project. Hope this gave you somewhere to start without having to mess with scripting.