Welcome to RC
what size pumps would be recomended
Personally I would go large and throttle it down if you need to, that way you have flexibility for the future. Something in the 1500gph range. Also I would tree off of the return line into a a few ball valves so you can easily take water out of the tank for water changes. Maybe one out of the tank and one pointing into it. Also, pumps go down in output over time.
You hole/s need to be larger than the maximum output of the return pump
http://reefcentral.com/index.php/drainoverflow-size-calc
As far as placement goes it depends on your overflow design. I don't have one but Bean Animal overflow designs are good. External and drilled overflow boxes make for a nice clean look.
Sump design
Before you design your pump you need to pick a skimmer. It will determine how high you baffles are and where they are placed. Your baffle determines the water height in the skimmer section so it doesn't fluctuate. Internal skimmers need constant water height. This guy has awesome sumps.
http://www.melevsreef.com/allmysumps.html
Model f is a simple 3 chamber
I have a 140 half round and the largest sump I could fit is a 28 gallon. With the skimmer and return I could only go two chamber. I use part of the return side as a refugium with the plastic board I use for frag racks and a algae scrubber above it that feeds off a ball valve tee'd off the main pump. I have a separate smaller pump in the skimmer side that I use for my media reactor pump.
Most sumps are 2 or 3 chambers with the drain pipe going into the skimmer section.
Skimmer/Return
Skimmer/Return/Refugium
Skimer/Refugium/Return
The float for a auto top off would go in the return chamber
Now we got FishyBizness on our side too
