If you are on a budget, you could get the hydor mini heater. They keep the tanks a pretty even temperatature and they don't get too hot.
You can leave it on all the time and it works well. I have one too. :)
They don't cost much either, and if needed you could wait till it's on sale to buy one or buy one that's less online. You can also get a a cheaper tank if needed, it is still very good quality; petsmart and petco carries critter keepers. The large one is 2.0 gallons, and extra large 2.5 or 3.0 gallons. I think the heater would be good for them too, just keep it in the upper 70s (76-79?). You would just need to get a smaller filter for it, but betta fish don't have to have filters, so that would help too. A turkey baster works really well in cleaning gravel, and it doesn't cost much either, only 1-2 dollars. For lighting, you could use a lamp with a fluorescent bulb beside the tank. It works! Just make sure the light bulb is the right watt for the lamp.
At least you have some betta fish though, some people aren't allowed to have any pets!