I love this heater (so do a lot of other people here): http://www.amazon.com/Hydor-THEO-Sub.../dp/B0006JLPG8
It'll work in up to 7 gallons according to its specs, but I'm using mine in a 10 and it's fine. The general rule is 5W of heater power per gallon, but you can use less than that. Depends how cold you keep your house. :P When buying heaters, always go for an adjustable one and make sure you put in a thermometer with it so you can make sure it's heating to the appropriate temperature.
As far as filters, I think that waterfall filters are the easiest to baffle, since bettas don't like current.
I baffle them like this:
That's Fluval filter foam/sponge (which comes in a box for like $2) rubberbanded around the filter, and pushed into the waterfall. The idea is to displace the water over a wider area, so you want the foam to touch the water. There are other ways to baffle, but I like this way.