Here's the rule of thumb I've learned from doing tons of research. Light is your most basic requirement and necessity. 3wpg is optimal. Some plants WILL do better with more, but will still grow just fine with 3 provided they receive supplementation. IE: CO2, ferts, and some nitrate to take in.
Iron is the most basic element which plants like and generally require the most of. Which is why seachem flourite and eco-complete engineered their substrate around it. Without such a substrate it will be important for demanding plants to receive supplementation of iron and other trace elements either a) through the water column with powder or liquid ferts or b) root tabs or c) both while making sure to always follow instructions and dosages.
The only test kits I use post-cycle are a kit for nitrate, and ph. There are also kits to monitor your CO2 in ppm I believe but I just prefer to use the bubble count method (watch the bubbles as they leave the hose and count).
Carbon Dioxide supplementation is generally not necessary for the -survival- except for the most demanding of plants, which are generally not readily available to aquarists anyway. However with many plants such as carpeting plants (dwarf hairgrass, microsword, certain mosses, dwarf baby tears, etc etc) it will drastically increase the replication, livelihood, and beauty of these plants and will aid other plants who require high light in photosynthesis.
Basically all I'm trying to say is that as long as you're meeting your light requirements and your plants are receiving some trace elements they shouldn't be dying. Ferts are also more important in your tank because you only have several fish in a 55gal which means that there isn't a lot of nitrate for your plants to be taking in.
Eventually when your tank has some time behind it and you have your fish stocked the way you want to you'll find a good balance of overstocking your fish (as per the 1" fish per gal rule) and creating enough nitrate for them to thrive while also making large (up to 50%) weekly water changes to replenish the trace elements they take in to grow. Perhaps I'm going a bit too far ahead though. :)
Snails by many are regarded as pests. My personal experience is they're a great help to the aquarist as long as you're not overfeeding your fish. An abundance of food will cause snail populations to explode, literally. If you're careful not to overfeed your fish they will always maintain a manageable population and will do more good than harm as all the common hitchhiker snails that often come into your tank with plants (it was quite a surprise for me when I saw my first one) won't actually eat plants unless they're dying/melting/rotting and will actually consume a large amount of algae.
As far as where to get them...just try to research what you're contemplating getting. Terrestrial plants can be considerably more challenging to keep in aquaria but are sold to be kept as such because a few people out there basically found a way to keep them fully submerged and alive. There are a good many sources on common plants to keep and although much of the information varies slightly (as it does with fish) generally it all averages out to a fairly round number relatively close to both ends of the spectrum. There are also many places you can buy live plants online from and thankfully the shipping charges for plants are much less than fish....just use google, its your best friend.
I hope I gave you more answers than questions. lol