[glampka]

Hi Dan,
The only stupid question is the one that isn't asked!! Smart move - keep giving the wife some credit for allowing you to do this.
I'll start here & will no doubt PM you as you've asked a ton of questions that will take some time to answer.
Acrylic lid - do away with it if at all possible. You already mention the gas exchange issue. A more important one is the tank temperature issue especially with the MH lights. The lid will hold in the heat. You also don't want to put a venturi on 1 of the ph since you'll have all sorts of salt creep problems to deal with.
Sump - IMO you woould be better off putting in a sump with a good size return pump. Trust me - your wife is not going to like the looks of 7 powerheads in the tank nor will you! You can build a manifold to supply current to the tank from the return pump. 7 phs will also add heat to the tank. You can put carbon in the sump if you like. Canisters become a big nitrate problem because people generally don't clean them enough.
With the acrylic tank you can have an internal overflow built right into one of the ends. The overflow takes water from the surface of the tank removing the oily film that tends to build up there to the sump where you would have your protein skimmer. The skimmer then removes these proteins from the water. You want to get a skimmer that is rated for doulbe the size tank you have, in your case 300 gallons. Take a look at ASM & AquaC. Both are highly rated. Don't skimp on your skimmer. It's one of the most important things in your tank.