Yes you can buy a used heater providing it works. Tree frogs like crickets and other gut filling bugs which you can buy from pet shops. Don't forget to calcium dust them at least once a week. Ten gallon should be ok for 2 frogs but they prefer a bit bigger.
One frog will be ok on its own though. I only have one golden tree frog and he's happy.

Have fun with your new frogs, they're great animals to keep.