Drainage layers are builders choice really. They both do the job. Hydroton is heavier. If you plan on Ming the tank around go wit egg crate.
Peperomia caperata oly grows 8" tall and peperomia verscheffeltii grows about 6 " tall. Both thicken out and widen so they should thrive in that size enclosure so long as the soil drains well and light reaches them. Which brings me to lighting. 6500Kelvin is great color spectrum and you'll need about 1200-1500 lumens to penetrate 20 inches to the bottom although you can get away with a little less if you don't mind plants not coloring to their full potential. The T8 you're planning on is more than adequate. I found full spectrum t5 grow light with fixture and reflector for 30 bucks on eBay. Shop around.
3 way opening lid sounds impressive. I'm assuming front opening, tilt slide and lift out? Screen versus glass will depend on humidity retention and requirements. As for water feature, I think removable is easier to clean, but less natural looking. And vice versa. You don't need to glue glass in place. Some use a pump hidden by cork. There's tons of ways.
What type of frog are you housing?
And finally, don't forget to take lots of progress pictures. It's fun and rewarding. When someone says how'd you do it, just show them..