Congrats, Tomas! A 25 is a nice size to work with. What are you hoping to keep?
With a 25 gallon, 20 lbs of gravel should do you well. If you buy it in 5# bags, only open 2, then see how the tank looks. Add more as you see fit. Are you hoping to use live plants, or
Plasticus americus for the time being? If you're just using plastic/silk, you only need enough gravel to anchor them to the floor (3/4" - 1")
I agree about not scrimping on a heater. You should do fine with a 100 - 150 watt heater. And the Whisper filters are very quiet and reasonably inexpensive. Don't forget to price them out online and see if anyone is offering free shipping or if you can pricematch at Petsmart etc. It never hurts to ask a store if they pricematch -- the worst they can say is "no."