If you're starting a reef (you mentioned that you want corals) I'd say that a skimmer, more LR, and better lighting are some things that you should look in to.
you dont have to run a skimmer with a sump. There are skimmers that are HOB (hang on back), and many models do a great job without a sump. A sump gives you a greater total overall water volume, a place to hide some equipment (heater, etc), and usually a place to put a skimmer.
Almost all corals will require better lighting than what you have. At a minimum I would look in to a power compact fixture, but beware because you'll most likely want to upgrade from PCs later down the line. You might as well invest in something nice right off the bat, instead of making a purchase that will eventually need to be upgraded.
What type of filtration are you running now? Most people that keep reef rely on lots of live rock and a deep sand bed for filtration, occassionally supplemented with additional filtration including wet/drys, canisters, biowheels, etc. There are pros and cons to each type of filtration system, so do you research before you purchase.
