300 gal setup

You can run both a wet-dry and a refugium/skimmer system, but it's really not necessary.

I agree you'll want more than a 30gal tank for a sump. For a 300g DT I would consider 60-90+ gallons for the sump. You're going to need a big skimmer for that big tank, and a refugium is all but mandatory to keep many species of corals and fish healthy and thriving. No way you can fit that in a 30 gallon sump. Bigger is always better when it comes to refugiums. A local here has a ~12,000g FO/shark tank built into his house with a 2,000g sump. You can swim in the sump.

You haven't mentioned if budget is an issue, but with a 300g tank and the desire to "do things right the first time" I suspect you're in this to win it. I would look into a Nepture Apex or AquaController system to monitor and automate many things. The complete top-of-the-line setup is around $700 for that, with lots of add-ons of course, so the sky is the limit. With the Apex you can monitor your tank via the web or iphone app.

-Doug