A 10 gallon is plenty for a betta. Heck you could make a nice community tank out of it. Throw a couple neon tetras or somethin in there. As for decor. Gravel is fine, I don't think they mess with it too much. I have a 100 W heater in my 12 gallon (I know overkill, but hey it works), so if you still have some sittin around you should be fine. They make much smaller ones (50W, 25W), and they're usually rated for tank size. (I think a 50 W is for... a 5 gallon maybe? not sure). I like to overrate my heaters, buying the next step or two up, but other people like to underrate their heaters, and get two smaller ones, in case one malfunctions and goes haywire, it can't over heat the tank on it's own.
Oh yeah, back to decor. Sometimes bettas like to sleep on broad leaf plants, so if you're going to go fake, maybe get something similar? Also, I'd be really careful to get soft plants, ones that won't snag the betta's fins. They really are easy fish to take care of, and beautiful too.
(Oh, if I sounded condescending, I didn't mean to, I know you're experienced with tanks/heaters, I just like to explain myself for when someone comes and reads this thread 2 months down the road.)
EDIT: Oh yeah, I wouldn't go for any of the "betta" specific dechlor or anything either. I've never heard of anyone using it, and why would a dechlorinator be fish specific? Stupid marketing ploy I think. You DO need betta food though, as regular fish food isn't the best for them I believe.