Why do you want to modify? I read of too many people here who talk about modding a bass before I is even in their hands.
Unless you are doing a build from scratch, buy something as close to what you want as possible. Then play it. Determine what (if anything) you think need a improvng. That will put you in a better position to ask a better (and more specific) question.
for example the Nate Mendel Fender Precision is a really good well made bass, but I know for example the Creamery 58 P pick up is way better than the stock one.
Just an example....
I can categorically say that I now prefer to play them stock, or mebbe with some easily reversible mods.