Doesn't hurt to invest in a skimmer. I would get a CPR brand for your tank. You can also do a couple other things. I also run mechanical/wetdry filters with my tanks just for extra filtration and circulation. A skimmer is all you really need. Especially if you have alot of live rock. That rock is essentially your filtration. Couple that with a clean up crew for the large stuff and thats all you need. The skimmer is necessary for the organics suspended in the water. It effectivly skims it out hence the name.
You could also invest in a sump. This will add further filtration and increase the amount of water in the tank which always adds to the stability of things. With a sump you can create a refugium which becomes a food supply. Or you can place a skimmer in there and carbon every now and again. It also further aerates the water. There are a number of things you can do with a sump...
Your choices are largely pesonal and financial so its up to you
