Hi and welcome!
If you are planning on hanging the skimmer off the back of the tank (HOB), the two best brands are Aqua C (the Remora Pro is a good HOB skimmer) and Octopus (which has a few different HOB models). The Fluval may lead to more water quality problems than it's worth down the road. Those types of filters generally don't do well for saltwater setups.
The top compartment of the SeaClone is supposed to fill with dirty brown foam, which is the gunk that is pulled from the tank. When it fills up, you empty the cup down the sink or toilet or whatever. Problem is, the SeaClone doesn't really do a great job of filling up that cup in the first place, but it will be fine for a new tank that is your size until you can upgrade skimmers.
I think the biggest thing you can do from the start to get your tank headed in the right direction is to NOT use tap water. Tap water is one of the worst things you can put in a saltwater setup.
You should only use RODI water. You can either buy a RODI unit to make your own water (they run about $100 and up), or you can buy RODI water from Walmart, the grocery store, the fish store, etc. Just look on the label that it is prepared using "Reverse Osmosis". Some grocery stores and Walmarts will let you bring your own containers and fill them up at a machine.
The cycle for saltwater is very different than freshwater. You're looking for 3 parameters. First, ammonia will rise. Then nitrite will rise. As ammonia and nitrite drop, nitrate will rise. Once ammonia and nitrite are both back at zero, your cycle is considered complete. Then you should do a water change to bring your nitrates down, and then it's safe to add your first animal/s.
Do not add any livestock to the tank until it has completely cycled. In saltwater, cycling a tank usually takes 3 weeks or more. So patience is key.
I think that's about it for the basics. What animals are you thinking of adding? Are you going for a fish only tank? Or a reef tank?
In either case, you will need some live rock. You can order live rock online or buy it at a fish store. 1 to 2 lbs of rock per gallon is the best. So you'd need 45 to 90 lbs. Live rock will serve as your tank's main source of biological filtration and it's one of the most important components of a saltwater tank, especially if the rest of your filtration isn't great.