They are about $60.00 you don't have to get exactly that kind.
It just controls your heaters more accurately.
You can add it later after you start to have some money invested in your live stock.
Cycle your tank first.
In your list you have your salt listed as Epsom salt.
Make sure to use proper reef salt. You will have to buy this from your LFS.
PS two heaters are better than one incase one fails.