you will have a LOT of fun with the 2.5G.
one of my favorite size tanks... for me I have 13W, DIY co2, AS with HOB filter.
enough to grow glosso, and fissidens well. not too fast, not too slow
i had 23W before, with 2 2L DIY Co2... i had m.p. in there which grew crazy fast!
i got some algae so i changed light intensity and duration of light.
I don't know if 2.5G is enough for 1 betta. Def better than the 1 gallon but i never kept a betta before.
For my filter, I got ceramic rings, small bag of purigen i made and those foam filter that you put into Aqua clear filters. I just cut the foam up to fit into my filter. But the purigen..works wonders. My water is seriously crystal clear, I highly recommend it.