On a 150 (I assume 6x2x2) I would be looking at a minimum of a hole to carry 40mm pipe for the drain.
However if you are thinkinf of a closed loop, then you have more options. A number of people do not like a huge amount of water to go through the sump and prefer most the turnover to be in the tank (via powerheads or Closed Loops).
If you followed that you should be fine with a single 40mm drain and a pump around 2,500 to 4,500 LPH for the retun. Then, to get a better overall turnover in the tank you can have the CL (or even better a Tunze system) to ciculate thet ank and keep it all moving nicely.
You could decide to go all out and have everything run off the sump, but then you would need a 12,000 LPH pump and at least two 50mm pipes for drains. The problem with that would be the space used up by the plumbing and the difficulty in getting the return to direct water all around the tank to avoid dead spots.
My favorite system (to avoid high running costs) would be to have an Eheim 1262, or 1264 as the sump pump with 40mm sump piping and throw a pair of Tunze 6100 in for the water movement run off of a controller. The Tunze would be about £500-£600 to set up, but would have insanely low running costs (the maximum each pump would pull is 45W, so from 90W you would be getting 24,000LPH). The controller means the pumps can be set to ebb and flow and create a more natural system, they even have the ability to sense whether the tanks's lights are on so that the flow is reduced at night.
A cheaper alternative to the Tunze are the Seio pumps, though these are not controllable and are made by Rio. Any time spent on the internet will see that most reefers are very careful about putting anything Rio into their tank after a large batch of powerheads failed catastrophically (leaking toxins into the water and destroying livestock). However these are meant to be completely different and that should not be possible with these, though they are more expensive in the electricity than the Tunze (but far cheaper to buy initially).
If you went closed loop then you have all sorts of options. One of my favorites is the Sequence range of pumps. The Sequence 10000 runs at around 9,500 LPH for only 95W and can pump up (so you could mount the pump in the stand). A pair of them would create some amazing movement (the larger pumps seem to lose their efficiency). Expect to pay around £150 for these pumps (they have recently been refitted with an improved motor section, so prices may well start to fall in the next few months).
The best (most efficient) closed loop pumps are usually external ones, and they have the advantage of venting motor heat to the air, rather than to the water.
Hope all that helps, and I haven't bored you.
Andy
