First thing I should say is that there are many different ways to set up a tank, and you'll probably get 10 different answers from 9 different people. There are cheaper ways to set up tanks, but cheaper does not necessarily mean "cheap". Saltwater is an expensive hobby, and depending on whether or not you want to have coral or not will really be the deciding factor on $$.
In a nutshell, if you go fish only, you won't need to be as picky with your water parameters (nitrates mostly.) For that reason, you could probably get away without a protein skimmer. In addition, your lighting won't need to be much so you can save $$ there. The $100 you mentioned for lighting is for fish only - a lighting setup for a reef tank with corals would be much much more than that.
Yes... you have several different canisters that could be used for filtration. But most folks in saltwater kind of shun the canisters and go with live rock because of the constant maintenance the canisters require to keep them clean. Live rock will house the beneficial bacteria that takes care of your nitrogen cycle (converting harmful ammonia and nitrites to harmless nitrates). You need something to house that bacteria, and it'll either be live rock or the sponges/bioballs/biological media in your canisters. Live rock also gives the tank a more "natural" look, in my opinion. Without rock, you just have fish and water. In my tank, the fish love to hide in nook and crannies of the rocks and generally swim around and pick stuff off it to eat. It also gives the more timid fish a place to hide if they feel threatened. It's fun to watch my Bangaii Cardinal chase a chromis from one end of the tank to the other, weaving in and out of the rock towers... with the chromis eventually ditching him as he doubles back behind a rock pinnacle.
A skimmer attachment to a filter isn't the same as a protein skimmer. The main purpose of a protein skimmer is to take dissolved organics out of the water before they can break down into nitrates. Skimmers do this by basically forcing a bunch of air into the water, making foam, which then rises up a tube and is collected in a cup. The foam will trap the nasties and export it from your tank. Think about the foam along the surf line at the beach... that's what a skimmer does.
Skimmers are kind of a must for reef tanks, but for fish only it becomes a personal decision as to how much maintenance you want to do and how high you want to run your nitrates. A reef tank will want to run with undetectable nitrates, but it's not unheard of for fish only tanks to run with 20-40ppm nitrates. The only drawback there is that nitrates will fuel nuisance algae growth, so there's a drawback there.
In my opinion, you get what you pay for when it comes to skimmers. There may not be a big difference between a $200 or $300 skimmer, but there's a world of difference between a $100 one and a $300 one. More $$ doesn't equal great, but cheap is normally a good sign of "a waste of money".
I wouldn't go with crushed coral - especially if you skip a skimmer and live rock. You need some type of substrate, and "crushed coral" is often recommended. In my opinion, sand is a better option. Crushed coral tends to trap debris and elevate your nitrate levels. Sand allows you to utilize snails to keep your sand bed clean, and turn it over.
Good thing that you're thinking all this over and planning before jumping in head first. Many folks don't, and I think they probably sink a lot of money into this hobby before realizing they shouldn't have. Which brings me to mentioning watching Craigslist or eBay for equipment. A lot of people might be looking for some extra cash this time of year and selling off that unused SW
equipment that's lying around in the basement might work in your favor.
This hobby isn't cheap, but it's pretty rewarding.
Welcome to AquariumAdvice, and keep asking questions!