A little more info on the repaint/retool thing:
you don't have to worry about repaints, as any changes you make to a part with a certain mold gets applied all other parts that share that mold. Ed's point is that if you're modifying a part with mold #2, and that mold was later retooled into mold #3, then mold #3 won't inherit any changes you made to #2, so you would have to manually apply your changes to at least one part that uses mold #3. Wow, that's really hard to explain! Just do your best, and as always, we'll be reviewing any changes you make, so no worries.