You can do almost anything that you want, can afford or will fit in your house/ stand.
I have a 125 gallon tank and I have a 20 gallon tall glass aquarium as my sump and a ~35(?) gallon clear plastic rubbermaid type tub as my refugium. My fuge has 125 lbs live sand and Live rock rubble. My sump houses my heater, skimmer, baffles to reduce bubbles and my return pump. I also have an automatic water top-off that feeds into my sump, to compensate for evaporation.
I would recommend keeping the sand and rock out of your sump and just housing your equipment there. Sand can really do a number on your pumps and equipment. I will save you maintenance trouble in the long run, and really, with a fuge that big, you dont need to bother yourself with more material in your sump. Keep them seperate, you will have better success that way.
I have external overflows. I have three actually. I originally had two that just barely kept up with my return pump. I added a third one that just goes straight to my fuge. The other two go directly to my sump. The fuge is sitting on a metal shelf (from the hardware store) in my stand so that is elevated a bit above my sump. The water gravity flows through two bulkheads into my sump.
I think that it is ideal to have the water go through your fuge before the skimmer, so that food from you tank reaches the sand bed creatures in your fuge. You also want the excess nutrients to reach your sand bed so that your DSB can process them properly. Also so that the excess nutrients can filter through the algae in your fuge, remove more nitates, etc.
You of course probably do not want all of your water flow cycling through your fuge, you need this area to have slower water movement. People adjust for this in various ways, usually involving a tee and a ball valve off of a overflow drain line or a return line. I went for a diiferent approach with my extra overflow box.
How you set yours up depends alot on the available space you have. Be sure to consider the plumbing aspects such as bulkheads placement, tank/ tub drilling, pump placement (submersible or external), etc.
If you have that much $$ to spend on a skimmer, I would go with a Euroreef. They are amazing from what I hear. I can give you some links about them if you want. A good skimmer goes a LONG way in maintaining your water quality.
Good Luck,
-Christine
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