[Edi]

I suggest doing as Edratman does. Easiest way to tackle that problem. Would be a waste of effort to try to maintain some kind of library because there is no real way of indexing it unless you want to go to a lot of trouble to do it and for no really useful purpose either.

Creating a new unit from scratch is really fast if you've done it a few times. Spells, more involved and difficult, which is why existing mods are the ones you should look to first.