I know several people that have (or formerly had) a 55g reef tank with no sump. I find a 55g to be hard to use as a reef more so because it's so narrow front to back, making it more difficult to have any sort of creative rockscape. I wouldn't be so concerned about fluctuations in a 55g... People successfully keep reef tanks under 20g pretty regularly now. But, I guess it all depends how diligent you are and how a proper maintenance routine fits your schedule. I keep a stand alone 15g tank up and running for the local club as a frag tank with no real issues, but it has only a single fish in it. I try to do water changes every two weeks, but the daily top-offs are really the hardest thing to keep up with.
If you want to progress to a reef down the road, I would set it up now as much as possible like you would a reef tank, minus the reef lighting. I would encourage you to get a sump, and to DIY one yourself. The cost of a custom or pre-manufactured sump is ridiculous for what they are. Since it's a 55g, I would imagine the glass is tempered, which means no drilling. You would need an overflow box to move water to the sump. I would not suggest a CPR overflow or any overflow box that requires the use of an Aqualifter pump to operate. For about the same price of a new CPR overflow box that would require an Aqualifter pump to function properly, you can buy a
LifeReef u-tube overflow box that is guaranteed to not lose siphon during a power outage (called pre-filter / siphon boxes on their website) and requires no other equipment to function. You can choose whatever return pump that fits your needs for that height you'll be pumping water from the sump back to the display tank, prices will range up to a little over $100 online, depending on the brand and model you select. It's important to note that if / when you buy or DIY a sump, be sure you know how big the area is to hold the skimmer and make sure an appropriately sized skimmer will fit into that area. The last thing you want to do is buy a skimmer, only for it to not fit into the part of the sump designed to hold it.
When chosing a skimmer, keep in mind that the rating on the skimmer is for a lightly stocked tank. Let's face it, out of the dozens of marine tanks I've seen (both FOWLR and reef), I wouldn't categorize more than one or two of them as lightly stocked. Generally, when shopping for a skimmer, if you cut the rating in half, you're much closer to what the skimmer can handle with the stocking rates most typically seen. For example, if a skimmer is rated for up to 120g, it would truly be undersized for the average tank that's larger than 60g or so.
Since you're starting off as FOWLR, the amount of flow in the tank doesn't need to be as high as it would in a reef tank. You would still want at least a couple of powerheads for flow to keep detritus suspended, but not as many as you would if you were looking to set up a reef.
From there, it's pretty standard equipment... basic lighting to see your fish, a heater or two, thermometer, and so forth. Depending on how much live rock you put in the tank, you may want or need a HOB filter or canister filter, but both of those can be avoided with enough live rock in the tank (1 - 2 lbs / gallon, which really depends on the density of the particular live rock you choose).
Good luck, and keep us posted!