My best advice for any newbie marine hobbyist whether they are starting a reef or a FOWLR is to get an over-sized skimmer of a good quality brand. You should expect to pay about the same amount for a new skimmer as you would for a new tank (minus the stand). To save some money, or to buy a better skimmer for the amount of money you have available you could consider buying a used skimmer. There are lots of good deals in the classifieds here and you're in a big enough community that there will be some selection too! For a 120 gallon tank you're looking at $300-500 for a decent new skimmer...or course the sky is the limit and you
could spend well over $1000 if you wanted the Astin Martin version!
Your next most expensive buy will be the live rock. I suggest you buy this "used". Google some pictures of Aiptasia and Majano anemones to familiarize yourself with these pests that can be a real pain in the butt to get rid of - take pictures with you when you go to look at rock. Beware of rock that the seller says has baby bubbletip anemones as they aren't usually bubbletips, and are usually Majanos. "Used" rock will cost you about $4/lb compared to the $7-12/lb you will pay at the LFS. Rock from the LFS will come with its own fair share of pain in the butt as well. Dry base rock is
not live rock, and will take several months to become live rock. The more base rock you use the longer it will take. Live rock is your main biological filtration, so if you buy base rock you will need to stock your tank much, much slower.
Research EVERY fish BEFORE you buy it. Don't buy the fish based on what the person at the LFS says about it. Not
all LFS employees know what they are talking about, and sometimes they make mistakes too. Add the most aggressive fish last.
A 120 gallon tank is a tad small for the most exciting FOWLR stocking options, but if that's what you have, and that's what you want then there is no reason why you can't make it work. Be aware that you won't be able to add any of the large Angels to the tank other than a Majestic Angel which stays smaller.
Last piece of advice, prepare yourself to only be able to add 1/4 of the stock to the tank as you would expect to add to a freshwater tank because then you won't be disappointed when you are told your tank is full.