Hey Guys.
I would also suggest an external pump. When figuring head pressure, you have to calculate every foot up as well as every 90 degree bend as One foot of head. A standard 125 stand is about 30'' from the floor, and the tank is 24" tall. That's about 5 foot of head. The 125 has holes drilled in the overflow box to put the 3/4" return line, which then leads to a single 90 elbow that directs the flow into the tank.
If you are planning on a high flow aquarium, a Sequence or Dolphin external pump are good options. The heat of the pump is mostly transferred to the air, unlike a Magdrive that transfers the heat to the water. I wouldn't recommend using a Magdrive as an External, even though some have done it.
It's good to get an oversized pump. You can always turn the pump down if it's too much, or T it off near the pump to run something else down in the sump like a skimmer. But you can't do anything if the pump is not enough.
Another tip is to oversize the main manifold that pumps up through the floor. The wider the pipe, the less friction that is created inside it, letting you get the maximum flow out of it. If the pump output is 1.5", you can use a 2" line to pump up through the floor. Then, once it's through the floor you can T it off. I used a 4 way cross to make 3 outputs instead of the two you get with a T.
And you can run a pump off the bulkhead on the Rubbermaid tub. I believe the hole is 1.5". We used it for both sumps at Pet's Plus, and it worked fine. You can occasionally run into leaks with the threads on those tubs.