The think about my stand was, I built it for a 120 while I still had a 29. I've upgraded since (too good a deal to pass up) and will be bumping up to a 120 soon enough.
With the smaller tanks, I did ventilate through the top sheet, and now the plumbing runs to a sump in a dedicated fish room, so it's empty underneath.
Since yours is built to the exact size of the stand, I would consider incorporating venting or fans on the sides. It will help keep a more stable pH, and keep moisture down.
I have a Super Reef Octopus HOB
Skimmer now, the XP1000s, and it works great. I'm looking at either a SRO XP3000 Ext or XP5000 Ext for the upgrade. When I get the 120 I will net about 200 gals but in about 2 years I'm adding a 240 in the main floor (both tanks same sump) to a net volume of about 375 gal
. I guess my point here is, I think the SRO is great bang for your buck.