A 50w heater will make the tank about 10 degrees above room temp. A 100w can bring it about 15 degrees above. Depending on how cold you keep your house, either would work but the 100w is longer and will need to kept at an angle (I do that normally with mine anyway it's fine).
I like Aqueon Pro heaters lately. I also like Jager brand. I have used Marineland Visitherm a lot successfully in the past. Not fond of much else.
You don't need lighting but if you wanted to get a good you could. I started with this one: http://www.petco.com/product/112178/...ium-Hoods.aspx
Otherwise just a clear glass lid would be fine: http://www.petco.com/product/107312/...ium-Cover.aspx
Well a lot of fish that can be kept with bettas are school fish and should be kept in schools of about 6, like neon and cardinal tetras, as well as corys. If you want only a couple tank mates you could consider an apple snail, or a couple of otos. The snails have a higher bioload and the oto are very sensitive fish so make sure your cycle is well established and test often to make sure things stay okay. Otos are also happiest in schools but they'll be okay in sets of two. They'd be happier if you added 4 of them. They will need to be fed algae pellets or wafers. They have a lower need for schooling than tetras who flat out require it or corys.