Welcome to the forum!
1. Most people here, myself included, will recommend at least 2/2.5 gallons per betta. Of course, the bigger the better! And that way, you might be able to split it later on for another betta...
2. Pellets or flakes designed for bettas is what they need as a staple diet. Most bettas don't like flakes, though. One of the best pellets out there is Hikari bio-gold, but I feed mine Topfin since it isn't available at Petsmart. You can also feed them frozen or freeze dried foods as a weekly treat. Just make sure the froze food is thawed and the freeze dried food is re-hydrated before you feed.
3. Most kits are better priced better than buying each individual item. Just make sure that you get a heater, since I notice that the starter kits normally don't come with one. Bettas need warm water!
4. Silk plants are great! Plastic plants are also okay, as long as they're not sharp enough to tear your betta's fin.
5. It can be whatever you want: gravel, sand, rocks, marbles, bare-bottom, etc. Just keep in mind large rocks or marbles are harder to clean because uneaten food and poop falls between the large crevices!
6. A lot of people here use Prime, but I've never seen it in stores myself. I bought a package of NovAqua and AmQuel, and i have no complaints. NovAqua is a water conditioner, while AmQuel is a water conditionner as well as an ammonia,nitrite, and nitrate remover. The only thing is that AmQuel has a very strong peculiar smell that most people find gross. I like it, though.