28L is a nicely sized tank for your betta! The tank itself looks nice too. Congrats.
Not sure how well a 25w heater would work, since they're made for up to 5 gallons...but I'm assuming that where you live is very warm, so you wouldn't need a lot of heat. So a 25 watt heater SHOULD be okay...buy a thermometer as well to keep track of the temperature. 78-80 F (or around 26 C) is the ideal for a betta.
People have different views on undergravel filters. I personally have no experience with them, so I can't give you any advice. Sorry.