I think you should buy your equipment slowly as you go, but be careful not to buy cheap/crap stuff the first time around, that you'll only have to replace later on down the road.
For example, if you want a reef tank with corals, lighting is very important. The lights you have for a freshwater setup will be just fine for a fish-only tank, but you will have to spend more on a new light setup if you want to keep corals. So I'd say to just start out with fish and non-photosynthetic inverts for now, save up money, and buy lights when you can afford them. By that time, you should have a good familiarity with your tank, and will be comfortable trying your hand at corals.
A protein skimmer is very important. I'd say if you are going to spend your money anywhere, to get a good one. If you get a good one now, you won't have to replace it down the road. Check out
www.aquacave.com, and look at the Reef Octopus line of HOB skimmers. You want one that is ideally rated for twice the volume of your tank. Octopus HOB skimmers generally cost $100 to $200.
You will also need to buy a couple powerheads (two will be fine) for water movement. If you have some from freshwater, you can clean them off and just use them. Saltwater tanks need a lot more water movement, so odds are you'll have to get more, or stronger ones.
You will need to fill the tank with rock (1 to 2 lbs of rock per gallon). So aim for 50 to 100 lbs of rock. You can buy it all as live rock (which generally runs $4 to $10 a lb.), or to save money, you can buy it mostly as dry rock (base rock), then add a few pounds of live rock to the tank. The live rock will seed the base rock, making it live eventually.
Lights, protein skimmer and powerheads are really the most essential and basic of the equipment you'll need. Anything beyond those is pretty much optional.
You will need to think about where your water is coming from. For a reef tank, it's a bad idea to use tap water. You should only use RODI water. You can buy RODI water at any Walmart or grocery store, or you can buy an RODI unit for yourself and make it at home (a small one will run you $100 to $150, larger and faster ones will be more expensive).
I'd say unless you are going to be buying your water from a store, an RODI unit is an important purchase.