Welcome to the fish world and this forum, hope you'll enjoy yourself!
If you can get a liquid test kit from API or alike. The strips are extremely inaccurate and while they show you nothing for NO2 & 3 it could well be you are having slightly elevated NO2 levels tested with the liquid set. They often (if not always) only then show readings at all when its pretty much on the verge of being too late already. Also the strips only last so long before the colors are faded and then inaccurate to, over time you'll test your tank water on & off and to keep rebuying the strips is more expensive in the long run then the test kit. Just make sure you can test for NO2, NO3, pH, KH and Ammonia.
A wonderful idea live plants, can only encourage you to do it. Not only will it look much better but also will act as a "bio filter" if you will and so it helps your tank, your fish and your eyes
To ensure proper plant growth you need a proper light, so check what bulb you have under the hood there (the usually have names printed on the sides of the bulb). It should be ideally a full spectrum daylight rated around 6500K for plants.
Then really its up to you what plants you like. I'd suggest for starting out to get a combo out of Stem plants such as Ludwiga, Vallisneria or Cabomba, these will grow fast while plants like a Amazon Sword rather grow slower but looks pretty. There's a good selection of plants with pictures here for you to consider HOME_PAGE_TITLE
This one also has lots pictures and care info Aquarium Plants
If your filter bothers you and you think its too loud, exchange it for something better. Being a 55g I'd rather suggest a canister filter then the one's that hook on the back they'll be less nosy and more efficient. My pers favorite I had for years and years on my 55g and larger are these from Eheim Model 2213, they're working great, zero sound, last forever and a day and are not as pricey as most others EHEIM Classic External Canister - Filtration & Circulation - Fish - PetSmart
You CAN exchange gravel if you want. That said and the wish to exchange the filter you have to know: All your necessary bacteria lives in your gravel & filter, so pulling both out would hurt your tank. What I'd therefor suggest, if you get a new filter, simply put the old filters sponges and other media in the new filter (so you bring over the bacteria there) give that a solid 4 weeks to all settle again, then pull the tank apart and exchange your gravel.
Hope this helps you for the moment