I am surprised to hear about the 1760 gateway to be good up to 30 channels.
I thought that it was only good for few analog lines.
About outsourced billing: The best thing that you could do. It will save you a lot of troubles when you buy the real billing system.
What you learn on someone else billing system will help you keep sales people honest when you start shoping for a billing system.
And that is a challenge!!!!!!
About using the 2611 as a gatekeeper:I found out that you could have a real good gatekeeper for less than $200 . Buy a MC3810, get the righ IOS, and you get a nice gatekeeper, Cisco.
You could not beat the price,even could keep a second one on standby and still be lower than $500.00 dollars total.