Yes you can have your tank drilled or you can go w/ a hang on overflow (build one or buy one ~~~> IMO if you build it it will save you a h*ll of alot of money)
I believe your not supposed to pull the water out by a pump I may be wrong though.... but I believe the best and safest way to get the water out is by gravitation and a syphon.
The 2500 would be fine to pump the water back to the tank you just have to make sure your overflow box is rated close to what the pump can put out so your not putting to much back pressure on the rio by a ball valve....
What all are you planning on putting in your sump/fuge?