New Bigger Tank Questions

[Rich-n-poor]

I have pretty much decided on a 72 gallon bowfront tank with one corner overflow. I would like some feedback on the following:

I want to use a sump, what size would you recommend that will fit under the stand and why ?

I want to add metal halide and reuse the 110 watts of powercompacts I currently have. I was thinking of getting a dual bulb setup like 2 -150/175 watt meatl halide

I will be usng a seaswirl and two hagan 301 powerheads for circulation

I will reuse my bakpak II for the time being until I can upgrade to an in sump skimmer

I will initially set up the tank with 4-5 inches of sand, then transfer the sand and rock from my current tank, I will cycle new rock in my old tank and add it to the new tank until I reach about 100lbs of rock

please point out any deficienies problems and oh yeah what the heck is a closed loop ?
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recession