A protein skimmer is not essential but it is very useful. It works by removing organics BEFORE they enter the nitrogen cycle, and thus reduces the load on your filtration system. A decent protein skimmer is not a cheap thing, but cheaper ones are rarely good performers, and this is one area not to skimp.
Some people will tell you that you don't need one for fish, only corals. This is not true - as fish are messy, it seems to me you have MORE need for a skimmer on a fish only tank as there is more mess. Now fish are more tolerable of shabby water conditions than inverts (generally), but you still don't want a high maintenance, dirty, algae ridden wreck.
Also filters do not remove trace elements - there is a lot of mythinformation in salt water. I would strongly advise that you read the Paletta book, and assume and plan that you are going to succeed