I’m not an expert by any means but I would shoot for a higher wattage. I have had a 10g terrarium with mosses that have done well for a few months though. The light you have is 1.5 watts which might not be enough for mosses, I found this one for a similar price that’s a bit more powerful (https://www.amazon.com/NICREW-ClassicLED-Aquarium-Extendable-Brackets/dp/B0191EWII2/ref=mp_s_a_1_2_sspa?keywords=16+inch+aquarium+light&qid=1565136350&s=gateway&sprefix=16+inch+aqu&sr=8-2-spons&psc=1)
Yes, not as easy as that seems. For beginners they always say larger volume is better/the solution is dilution... But, if you want to get a simple 5.5 gal going, you can definitely do it. I don't know where you are but in the states, they have kits, which I've only seen with a black plastic lid where the light is attached inside/under the lid. I got this once because it was on sale... and I ended up tossing the lid, buying a glass lid and light on amazon and it is a great set up now. And depending on if your fish like water flow, you might get a different filter. So, here's an example: Light from Amazon will fit on a 5 gal tank and is less than 20 quid): https://www.amazon.com/gp/product/B0191EWII2/ref=oh_aui_search_detailpage?ie=UTF8&psc=1 Get a glass top if you can but be sure it fits (over here the topfin set tank didn't fit the aqueon lid I got!). Don't know if you have Cobalt Aquatics 25 watt adjustable heater, but that's probably the best in that size that is adjustable. But I paid $40 for mine (although I just saw one on amazon for $25). And you can choose between a sponge filter or a hang on back filter. Pros for sponge are less turbulent water(unless you have really big bubbles)/Cons-it doesn't suck up any of the crud/mulm that might be in the tank... like I don't like the dust from the Catappa leaves (as they decay) I add to some tanks, so I don't like to use them in sponge filter tanks. Pros for HOB filter, greater flow, can overfilter a tank allowing for a slightly higher bioload (like I have a filter for a 30 gal on a 10 gal that has Endlers livebearers 'cause they breed so prolifically and I have to sell them back to my LFS periodically so it's overstocked). And it sucks up mulm and shedded needles from hornwort and cabomba plants (unless I put a prefilter sponge on it). Cons--if you have baby shrimp or fry they would get sucked up without a prefilter sponge. All 3 of my 5.5 gallons have HOBs ranging from an Aqueon 10 for the fish that like flow to a small Deep Blue Nano filter for fish that don't like flow.
Hope this helps and sorry if it was too long!
https://www.amazon.com/gp/product/B0191EWII2/
and if you want a timer with sunrise and sunset type fade in/out get this with it...