I think it's awesome that you're doing your research!
Your boy will be fine without a heater until then. As has been already said, just keep him in a place where there's not a lot of draft and wrap his tank in a towel at night.
The only two heaters i have experience with are the top fin brand and hagen elite submersible. The Elite submersible I have in a 3 gallon tank, and it keeps it at a perfect 79 degrees! I do want to advise you to test out the heater in a tank of regular water for at least 24 hours, before putting it in with your betta. I normally leave it on overnight, check the temp with a thermometer, in the morning to see how it manages the colder temperatures in the evening.
Also, especially if you're going for the top fin brand- after you're comfortable with the fact that it manages the temperature good, be sure to pour out some of the water that had been sitting with the heater into a separate container and smell it to make sure there's no funky smells. I've known people who have had problems with the Top Fin brand leeching stuff into their water. I returned a 25 watt Top Fin heater after it made my water smell like rubber. However, I do have a 50 watt Top Fin in my 5.5 gallon and haven't had any problems with it so far.
And welcome to the forum!