There are alot of substrates you could use for a planted tank. I personally prefer miracle grow organic mix/soil topped with black diamond blasting sand or some type of not to fine sand. Gravel and rocks didn't hold my plants down well enough when i tried them and i like the looks of the thicker sand better. You could do an inch to 1.5 inch of soil (sifted to remove mulch and rocks) then cap it with a 1.5 inch cap of whatever substrate you prefer.
Amazon swords grow pretty tall but for a background plant if you trim it and keep it from getting to tall, it would work for a year or so. After that it would probably be a little to big and tall for a 5.5. Other type of plants if you are going for a well planted tank would be -
Various types of hygrophilia
Floaters - Frogbit, dwarf water lettuce, greater duckweed, salvinia
So long as your snail is a mystery snail and not a huge apple snail, it will be fine with plants and won't eat them. So it's up to you if you keep it. Mystery snails do great in eating algae from the tank walls and keeping the tank clean. If you go the route of live plants, you will more than likely wind up with other snail types unless you dip all the plants well when you get them. I, personally, love snails - all of them.
Lighting is something you will want to look at. Unless you have a stronger LED, it's better to get some type of Lid or light that goes over the tank that you can put a flourescent light into that is rated 6500/6700k and that will grow plants very well for you. Lighting is the biggest hurdle to growing plants well. A 5.5 pretty much any lamp or hood that you can put this type of CFL light into will grow plants perfectly for you.
You can wipe down the inside, filter and heater with a bleach solution, let them soak for a day or so and then drain and sit the tank for a day or so in heavily declorinated water to get rid of the bleach residue. This will clean it and get rid of any lingering germs from the death of your other betta and get it ready for new inhabitants. It will mean you do have to re-cycle the tank so be ready for the water changes and spikey perimeters for awhile until it gets cycled.
It's usually best to use declorinated tap water for fish and live critters as the water has things in it that they need to be healthy. Distilled water removes these things and take the minerals they need out of it. If you are truely concerned about the safety of the water, you can throw it in a bucket and let it sit for a week for the next weeks water change or boil it for a few minutes, let it cool and then use it for the tank. I wouldn't recommend using distilled water permanantly for fish tanks.
If you have any questions when setting up feel free to PM me, be happy to answer questions for you as you run into them. Also you can watch for my sale threads, i sell clippings from our tanks from time to time and could find you a starter package of plants when you are ready for them. :)