Maybe a 15w? You might have to try with a couple of bulbs and see which one gets the best temperature. I'm sure I've gotten close to 30C with just a 15w. Mat should be ok in the short-term.
I wouldn't have both the heat mat and the bulb on at the same time, but if you're worried about night time temperatures the mat could probably be used for that.
You can get special magnetic ledges with food bowls attached. You'll have to look around for them though, or make them yourself.