Here, use this calculator to calculate the depth of your sand:
Also, the depth of the sand is based solely on your own personal taste. If you like a thin layer of sand, then go with 1-1.5". If you like it in between, go with 3". However, if you want the added benefits of a DSB
(Deep Sand Bed) then you need to go 5" or higher. I'm not talking about 4 inches in one corner and 5 inches in another... you will not benefit from a setup like that. It needs to be a consistent 5" all around. Most people just go with 6" all around to be safe. I'll be using a DSB
for my fuge on my current build. 6".
Anyways, i would advise against getting the sand, washing it and then leaving it in your tank. My concern is that, sand that is waterlogged never really dries out, especially fine sand. Therefore the water still in the sand will become stagnant and will collect bacteria. Your best bet is to get the dry sand and then wait until the day you plan on filling up the tank to put it in. Definitely rinse it thoroughly before putting it in your tank. I always rinsed it 5 or 6 times, then put it in, and then filled up my tank.
On another note.. HOB
skimmers are an eyesore, and on a big tank like that, you will definitely benefit from having a sump. Almost all tanks over 100G usually have sumps or built in refugiums. At least the ones i've seen.