(1) QT tank will help keep various nasties out of your display tank. That whole setup can cost less than a typical fish and can be broken down in between purchases.
(2) Figure out what your end goal is as far as fish and/or corals. Make sure you get a tank large enough and keep in mind that you will likely want larger, regardless of what size you start with. I regret doing a 75g this time. Should have just done 120g and am actually leaning towards larger. Bigger is better with saltwater for a variety of reasons.
(3) Research and ask questions here before buying anything. Post your equipment plans to get feedback and post your fish stock list to see what everyone thinks. You will get good advice here.
(4) Keep in mind that your LFS
is there to sell stuff and make money. Never assume that just because it is clean and the owner says they have lots of experience that they are always going to give the best advice. Some "old folk" are stuck in old ways of doing things and some places will give questionable advice. "It could work" probably means it won't.