Well, it depends. Some people like to cycle their betta tanks and others like to change all the water each week. Whichever way you go you will need a heater unless you live in the tropics, or unless your house is always heated in winter. Bettas are true tropical fish and will die in cold water.
This poses a problem because it's very difficult to find heaters for any tank smaller than 5 gallons. So unless you have a very warm house in winter or you want to get a tank at least 5g in size then bettas aren't such a good idea.
If you do decide to get a tank of 5 gallons it can be divided so that you can keep 2 male bettas. This is just my opinion, some others won't agree.
If you are getting a tank this size it's a hassle to do a 100% change each week so you may as well cycle it with a small sponge filter or small hob, something that doesn't create too much surface disturbance as bettas don't like strong water current.
It's up to you if you want to put gravel, or glass stones or whatever on the bottom but gravel will give you more area for breeding bacteria.
Hope this gives you some ideas.