Hello s.kemp, welcome to the MR family!
The first item you should get is a good RO/DI unit. Do not fill your tank with tap water.
Good idea to start with what you already have.
Will you be drilling the tank, or using an overflow hang on box? Or will you be using hang on filters to help with the filtration of your tank?
Has stated before it would be easier to give you ideas if you were to tell us in what direction you'd like to take your tank.
FO - fish only
FOWLR - fish only with live rock
Leathers & softies
LPS - long polyp stony
SPS - short polyp stony
Or a mixed reef.
What type of fish are you interested in keeping? Aggressive, non-aggressive, reef safe fish. So on and so on, and the list goes on.
We all for the most part recommend a Quality Protein Skimmer.
Wether it be a hang on, in sump, out of sump depends obviously on wether or not you plan to have a sump.
But no matter which setup you are planning a Skimmer should be one of your priorities.
Couple of power heads for circulation. Good quality heaters, and some sort of controller to keep them from cooking your tank.
ATO - automatic top off for the continues maintenance of evaporting water, which if not kept on top of will make the salinity in your tank fluctuate.
Lighting depends on what you want to keep. If FO, or FOWLR the lighting you currently have will be fine.
But if you want to keep corals you will have to upgrade the lighting.
Most important of all:
You need a ton of PATIENCE & RESEARCH!!!!!!!!!!!!!!!!!!!!!!!! And as you can see quite a bit of $$$$$$$$$$$$$$$$$!!!!