Hi Bibblewrap!
When you buy your snail, tell the store it'll be some hours before it goes into the tank. They'll put extra pressurized air into the bag. I've travelled snails like this for up to 8 hours (not by choice, got into a major traffic snarl so good thing I asked for the extra air) and they arrived home fine. Put the bag on the floor of your car -you can't fall off the floor - with towels, jacket, whatever to keep it from rolling around. Your snail didn't sign up for a roller coaster ride.
Nerite or mystery? Nerites clean up the tank, mysteries don't but they're entertaining with their explorations and sense of fun. One of each would be fine if your tank is 5g or more.
Get a good basic food like Crab Cuisine and put a pinch down every few days or whenever you see that the old stuff has been eaten and a day has elapsed since then. It does contain calcium. I personally have never used a cuttlebone but many, many people here do. The veggies are great - I give a bit once a week as a supplement. The packaged food is a basic nutritional building block and good for when you don't have the time to do veggies.
Vegetables need to be blanched in order to break down the fibrous exterior and become fully available for crustaceans. Blanching is quick and easy: slice the vegetable thin (or break off a piece of leafy green), boil it for 1 minute, let it cool. They'll need a chopstick or something to stay down where the snail travels most, otherwise it'll float on the water and have less chance of being eaten. Blanching is for leafy greens too, even lettuce. I personally avoid broccoli and cabbage; they break down sooner than the snail wants it, and become stinky in the tank.
Eggs: if you get one mystery snail, lots of fun. Two or more, you're almost guaranteed to get egg sacks. Don't ask. Not being into eggs, this would be a whole lot of dealing with egg ickiness for you. Nerites do produce the little dot eggs and they are hard to remove. I personally have found that by doing nothing (officially The Lazy Approach) they dissolve eventually. It does take awhile, but, as mentioned, they're impossible to try to remove.
Snails are very active at night. Don't be surprised if your food sits there all day and then the next morning it's gone and your snail is curled up for a nice long 12-24 hr snooze. Ah, to live the snail's life...