Voip Provider Basics?

I am currently managng a small Asterisk PBX at my work with a couple extensions and one trunk line to broadvoice. We would like to become small voip provider for the small businesses around us and I was wondering about a couple things.

-Is there a simple way of mapping one DID number to a group of users or maybe just another Asterisk PBX on a remote site? Also, if we were to expand, is there any bashg script availiable to add users?
-Has anyone had experience with obtaining the E911 service?
-How complex is it to switch a PSTN number over to a voip service?
-Has anyone had any personal experience with AstBill and is it enough?
-What would the best codec be to use in an environment with a symmetric 1.5Mb line?
-Is there any way to load balance between two Asterisk PBXs?