I haven't read the other replies as I'm short on time so sorry if I repeat anything.
I believe reading somewhere that this was a larger tank? 30 inch or so? If that's the case, you're gonna need a powerful filter, which does not mix well with a long finned Betta. I would never, ever, ever use a long finned Betta in a community tank (again). They get their fins nipped, their fins get torn up from the current/filter.
I wouldn't use a Betta to cycle a tank. It wouldn't even be close to enough to cycle a tank of this size anyway. You need something like Platys or Guppies to cycle a tank.
No need to clean it if it's been sitting out dry for a few days/week+. Gravel I would most likely toss or wash very thoroughly.
Do your research before buying plants, many places will sell non-aquatic plants and outright lie to you about whether or not they are truely aquatic. Goods plants are your Anubias, Java fern, Java moss (only when tied to rock/driftwood I hate it otherwise as it gets caught in everything), some Crypts, Bacopa, Ambulia, Water sprite, Hornwort, Wisteria and Hygro. Anything red is going to be harder to grow as it requires high lighting. The majority of the plants I listed are low light and some are slow growing. You're better off with your fast growing stem plants to start out with. Hygro (most varieties) is good for this. Ambulia, Wisteria and Bacopa too.
No doubt someone has answered the IAL question already.
Going along with the first thing I said, if you're talking about a long fin Betta.. Nothing except maybe Cories, Bristlenose, Platys and Swordtails, etc. If it's a short fin, basically anything that's non-aggressive.
Diet, varied. Look at the ingredients and avoid anything with 'filler' as the first ingredient. You want fish meal, shrimp meal, krill meal, etc aka MEAT as the first ingredient. High in protein as well. That said, mine pick at just about everything. Cichlid pellets, algae pellets, etc. I also feed frozen bloodworm, live blackworm and grindal worms.