Just chiming in with my 2 sense.....I'd go with the kritter Keeper (I have 2 of my boys in these - seperate keepers for each of course)
The lids are nice b/c they still let air through and keep the betta from jumping. As far as water changes go, ask the petstore how many gallons the keeper holds - mine are around 1 gallon a piece and I change them 100% every week.
For a substrate I use small glass craft beads, looks good and you can see through them so if you need to watch if your fishie is digesting his food correctly, you can tell =)
They're also big enough for a small plant (which my boy loves)
There are 2 methods for treating water, one is to let it sit out for a certain length of time, the other is to treat it with a dechlorinator. I use the dechlorinator route just b/c I want to be sure that no nasty chemicals are left in the water.
Good Luck with your new fishie!