Well it's good to see that you're trying to do your research before jumping into the upgrade.
An AC500 HOB fuge is a good idea, but you'll probably find the noise level a bit high unless you keep the tank topped up all the time. The waterfall effect can be quite loud if it drops even a couple of inches. If you add a piece of plastic to the outflow, then the water can slide down instead of drop.
I would save some of the old sand as is, wash out the rest thoroughly & add new sand if you need more. You can then add the old sand to seed the washed & new sand.
As for a 36" by 18" tank, you're probably looking at a 45g tank. However, many of these tanks have a black plastic support down the middle (both my 3 feet 65g tanks do). If you cut these on a new tank, it nulls the warranty. If its a used tank, cut away & get a piece of glass the same size to silicone in its place as a less light-blocking replacement support.
If you can get one or two large pieces of LR, that should be enough & they will also make good bases for your aquascape.
To save money, you can DIY a stand easily out of 2 by 4s. These may not be fancy but if done properly, they are very functional & sturdy. My stand for a 160g cost less than $80 & its roughly 82" long, 22" deep & 42" tall. You should be able to do it for $20-30 worth of wood & screws.
As for current, 2 Seio 620s should be fine, if positioned properly, especially with the return from the skimmer as well. I have a single Seio1500 on each 65g & that's plenty of flow. You can hold off on this purchase & see if its necessary later.
My 150w MH (Aqualight Pro) are not powerful enough for the 24" depth of the 65g tanks. However, most of my corals are on the LR structures & closer to the surface light.
If the new tank is going into the same place then you are going to have to set it up right away. I just moved my 2 65g tanks, although I let one sit running without corals or fish for a week due to a slight nitrite spike (may have been a contaminated test tube tho). It's usually better to set it up, test the water after a while & then put in the corals when everything tests fine. That being said, with your old LR going into the new tank, the risks are smaller that things will go wrong. Make lots of extra sw ahead of time & think of this as a massive water change. Have some prime available in case of a small ammonia spike. Save the old water to rinse off the crud on your LR, then you won't have any die-off from using fw.
Sorry for the essay.
Anthony
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