I also came from the reef world. I kept with the sump and haven't had CO2 out gassing problems. I'm probably using more CO2 than most - takes me about 4 months to go through a 5 lbs tank on a 75g aquarium. Haven't bothered to cover the sump yet also haven't gotten anything better than a glass diffuser - notoriously the worst setup for CO2. But CO2 is definitely the way to go.
I've got a heater, foam filtration, filter socks, bio balls in the sump. Media should be after the filtration. I don't have to clean my socks more than weekly.
I've got ADA soil, I'm happy with it. It was the most expensive part. I would check out DIY soil.
I use t5HO bulbs. 2 at 10,000K and 2 and 6700. Looks great.