People generally go with 10 times their tank size for a turnover rate. I have a 75 gallon tank and use a 700 gph.
You would need a turn over rate of around 1800 gph. I don't know for sure with a large tank, but I would imagine you'd be better off with 2 return pumps. And that way if one fails, you still have the other. But get that advice from someone with a tank comparable to yours
Do you have a reef ready tank or will you need to add overflow boxes? I believe a tank your size would require 2 overflow boxes. I'll warn you right now though - they are a pain in the butt - I wish I had mine drilled.
LG pumps are not quiet IMO. They also have to be oiled I believe once every 6 months or so. I have had great success with Iwaki and MAG. I turn my tank over about 15 times through my sump and then I also have two closed loops that add another 20-30 x flow.
It really depends on the animals you are intending to keep. If you are keeping softies, then you can do less flow. SPS require high flow. Determine the animals you want to keep and work your tank around them. HTH