It is illegal in some states to take seawater so be careful, some people do, do it but it is far easier making your own. There are lots of unwanted things in natural sea water.
Your LFS will know if the live rock is fully cured or not, you can get dead base rock but it takes longer to seed, you only need about 15lbs for that size tank, so it wont cost a fortune for actual live rock. Nothing happens fast in saltwater only bad stuff, so the thing here is patience.
That DI filter is ok but you would be better off looking at one of these
units which is a full RO/DI system, it saves you dragging water from either your work or LFS. They will end up saving you money in the long run. Get a TDS meter as well so you can see the total dissolved solids in the water, eg you want 0 TDS coming out otherwise you risk getting algae blooms. The cartridges need to be replaced every six months or if you have dirty water, sometimes soon.
The refractometer you have is good also, you NEED to get the calibration fluid though which is listed with that. It is is essential for that refractometer to calibrate it properly.
You dont need a UV sterilizer in that size tank, water changes are going to be your main way of keeping good water. In my opinion it is a waste of money getting one for that size tank.
Kelp, no they get to about 10" high and can spread like weed. Brush plant should be ok but needs to have supplemental iron and trace elements.
Nothing in freshwater can really be used in saltwater, you need to get saltwater specific supplements. Prime water conditioner is one that can be used but I would not use Flourish. You need actual Calcium
supplements once you start doing corals.
I would recommend this
as well for keeping your dkh at optimum levels.
In terms of water movement, one or two powerheads would be sufficient.
Skimmer is a bit of an issue, on a tank that size many people dont use them, but ideally if you are keeping corals then you should have one.
Plants, I would just get some Cheatomorpha for your sump, waiting until the tank matures some before adding anything to the display tank, it is personal choice but in a small tank it takes away viewing the fish, if they are in the display tank.
Sand or crushed coral as the substrate in the main tank, ideally sand if you plan on doing corals. It is easier to place rocks which will likely come with zoas if you get them eventually. You can use the crushed coral in the sump to help buffer the water.
One thing that hasnt been mentioned is how are you planning on getting water to the sump? Drilling the tank or with an overflow? You will need to work out water levels in your sump, so that it doesnt overflow with a power outage. Then you will need a return pump capable of keeping up with the overflow or greater. This is also why I suggested you get your own RO/DI unit as you are going to be replacing water lost to evaporation in the sump probably daily. This shouldnt be tap water as that can cause algae issues or cause your nitrate levels to spike.
Saltwater you want to aim for zero on nitrates and particularly phosphates.
Patience though is going to be key, it will take some time once you get the tank running before you can add anything so dont rush.