i just got my pair of cresties about 3 weeks ago, so im not a expert but I think I can help a little
1.) Definitely have the CGD as the stable, it has all the vitamins and calcium for a well balanced diet for Crested, They also have differant flavors you can try to see what one your crestie likes. But you can also feed them Dusted Crickets, Phoenix worms, or silkworms once or twice a week but no more then that. No veggies for these guys, I know you can make fruit like smoothies once a month for a treat for them, but I'm not sure on the Fruit that you can use.
2.) The least you can have for a single crestie is 12x12x18.(I think that converts to 15gal I'm not sure) There are many different tanks, you could get the naturalistic tank, there a little cheaper, But I personalty like the Exo Terras. They keep humidity well and have front doors and a door lock. I have a pair in a 18X18X24 and it seems like more then enough space, just never get 2 males, or a male and female if you don't want them to breed.
3.) If you get a young crested definitely go with just a plan paper towel or like beardies something they can not get impacted on when they are hunting. When they are older if you like you can do a naturalistic Vivarium and plant real plants with Eco earth(coco fiber). I have a live planted viv, but I want to keep the risk of them eating the Eco earth very minimal. I bought 2 feeder side ledges. They stick to your tank with magnets and there far raised off the ground with there GCD, they never eat on the substrate.
Hope this helps a little
