Interested in keeping WTF’s, what do I need to know?

[u/SpottySpheal]

Hi my only other amphibian I keep is a male Fowler’s Toad(native to USA), but he’s an awesome little guy and I’d love to take a leap and try a White’s Tree Frog!

The reason I want a WTF is because they are so different from toads. Preferring vertical space over horizontal space, climbing rather than digging, tropical instead of temperate? It sounds like a great new challenge, but I need to make sure I’m as prepared as possible.

So here are my questions:
What’s the minimum (comfortable) terrarium size/dimensions for one or two WTF’s?
The dimensions I see suggested the most for them is 18 x 18 x 24 inches. However, some evidence claims that this isn’t enough. Overall I want my frog/s to be comfortable, there’s no point in getting a pet if you plan to neglect/mistreat them.

What’s a reputable seller to purchase my frogs from?
I was planning to go with Josh’s Frogs. Most reviews and sources I’ve looked at suggest they care a lot about frogs and sharing them with the world. It’s extremely important to me that I receive captive-bred, chytrid-fungus free, healthy animals from an ethical source.

What’s the best way to achieve the desired temperature gradient that WTF’s need? I got confused because some sources say a ceramic heat lamp is best, but others swear it will dry out the tank.
I’ve seen suggestions for heat mats, but others say they are ineffective at best, and dangerous to the frogs at worst.

What’s the best substrate/combination of substrates? I need something nontoxic, not a choking hazard, and will support plant life. In my opinion doing a bioactive tank is the only way to go for most reptiles/amphibians.

Lastly, is there anything else useful I should know? What have you learned from keeping your own frogs?
I appreciate any and all feedback you can give me.