1. You need to decide on what size bulkheads you are going to use and then the corresponding hole size.
2. Most home improvement stores, Princess Auto and some of the online aquarium supply stores.
3. Can't help a lot on this one.
I would paint it after.
Check out this ad for an overflow box already made.
http://www.canreef.com/vbulletin/sho...light=overflow