On my previous tank, i used a standard 10G AGA aquarium as my refugium. It was a great asset to my tanks stability and excellent nutrient export. You can use a 10G tank for fuge, but if you plan to keep a skimmer in there, i suggest going bigger. A 10G tank is really small when you think about it, and i couldn't fit any skimmers in my small fuge. I suggest going with a 20G long, if you can. It makes for an excellent sump. Enough space for a fuge, skimmer section, reactor section, even a frag station, assuming you make each area equal.
The pump and overflow will depend on how much flow you want in the tank. I never drilled my 55G, so i can't help you with the overflow part. However, you could go with a 700GPH overflow system from glass-holes.com if you want to drill your tank.
However, what brand tank do you have? Is it an AGA standard 55G? If so, then you will need an external overflow box, as the 55G AGA's are made of tempered glass, and cannot be drilled otherwise they shatter.