First, if you're going to have a reef, pay the extra $3 - $5 per bucket of salt to get Reef Crystals instead of plain Instant Ocean.
Skimmer -- don't know any specific about that one, but I generally have heard good things about having a needlewheel pump on a skimmer.
Light -- A single MH bulb is not designed to cover an area 4' in length (1 per 2' is a pretty standard coverage area) -- will you be getting 2 of them?
RO/DI unit -- Shop around at other reputable online retailers. You can get a 4 stage unit for less than that and even a 5 stage unit if you like. I'd highly recommend staying away from eBay units, but others have had success with them, so I guess that's nothing more than my opinion. But, if you're ever on ReefCentral, there's always debates about the quality and effectiveness of eBay units versus typical online RO/DI retailers. What you should go with really depends on the quality of your water supply. Your tap water can be tested inexpensively (or even for free) if you are curious as to how good or bad it is. A simple TDS (Total Dissolved Solids) meter will give you a clue to generally how good or bad it is, but won't tell you anything about what specific chemicals are coming out of your faucet.
Personally, I like thefilterguys.biz -- I get all my RO/DI supplies from them now. I originally bought our RO/DI unit from melevsreef.com -- he sells 100 gpd RO/DI units (slightly more expensive than when I bought ours), with a membrane that doesn't have quite as high of a rejection rate as the 75 gpd DOW Filmtec membranes, but I didn't know any better at the time and our tap water isn't so bad. What that really means is that I've burned through DI resin a little more quickly than if I had a 75 gpd RO membrane, but that's something that's easily fixed once the RO membrane craps out in another 2 - 4 years.
Supplements -- A good quality salt mix and a regular water change schedule can greatly reduce or even eliminate your need for dosing all kinds of supplements. Our reef has been running just over a year now with soft corals and LPSs. The only supplements I've had to add are calcium (Seachem Reef Advantage Calcium) and alkalinity buffer (Seachem Reef Builder). There are other brands out there that should be just as good, I just started out using Seachem and like the results. Our corals are doing very well and I find myself fragging them often to keep them from getting too large. Generally, it's recommended that you don't add anything to your tank that you're not testing for. This is a good rule of thumb to follow when starting out, as an overdose of anything could potentially be disasterous to your livestock and / or your wallet. IMO, there's a lot of flashy products available that are designed to catch your interest more than they are to help your reef tank.
