Ya, you could do a betta in a 5 gallon. They're really good lookin, easy to care for fish for one, and over time they reallize who you are and kinda beg you for food. Only, dont feed it every time it does this, cause they'll do it even when they're not hungry in my experience.
As far as gravel, thats completely whatever you like the look of. If you're going for plants tho, i'd get sand. They make special sand for aquarium plants, but i've always had good luck with just play sand. You just need to make sure you rinse it in a bucket till all the dust comes out. You could also use pool filter sand for live plants if you wanted.
If you dont want live plants, you could get aquarium gravel if you want, but play sand is by far the cheapest thing you can buy. Its like 3 bucks for a 50 pound bag at home depot, but you'd only need about 5-8 pounds i think, but hey, more left over if you wanna start another tank!
As far as what plants to get, i'm not sure, but mine never ate any of mine, and i had too many to list.
for food, they'll eat anything besides flake in my experience. Brine shrimp, blood worms, are good, and they make special betta "pellets" i guess you could call them.
Depending if you want to grow live plants, you might need to upgrade your lighting. If its just the regular incandescent fixture that comes with most setups, then i'd buy one of those mini compact fluorescent bulbs that screw in. I think they make them in like, 10 and 20 watt ones, but i'd get the 20 for plants...
Also, what kind of filter do you have on there? I think that if you grow plants, you dont want too much flow, and besides, for bettas you shouldnt have a strong flow anyway. Also, i've seen where people will do an "all natural" filtration so to speak. They just let the live plants take care of amonia and all that, and just use the filter for a light flow. Not sure how it works, so i'm hoping someone else will pitch in on that one.
well, i'll end the novel here, but if you have more questions about them, just ask.
oh, and dont forget to cycle the tank first, but if you use live plants you might not ever see it cycle cause they take care of that by them selves.
hope this helps
mike