I have my UT heater stuck to the side of mine.. It's not that hot, plus I've never seen my frogs ever lay on it.
And now that winter is upon us I've been running a 50watt red heat bulb during the day along with my UVB and heat pad. I run the 25watt red bulb at night with the heat pad.
I think your 12x12x18 is good for just the one.

As far as the fake moss substrate and fern background, I have never seen these before.
But I really like the vines, I have one of those and one of a different brand, and I prefer the one that u have poster because the other one I have seems like icant get it wet and stuff falls off of it. My frogs like this vine that u have posted
I'm sure those other things are suitable for your frogs, it's just the matter of what u prefer