The temp/humidity sounds good. An 12x12x18 would probably be okay, it comes out to about 11 gallons. 20 gallons is normally suggested for one whites, so the 18x18x24 is closer to that. I understand that it is more expensive, this stuff does get expensive. I originally bought the 12x12x12 for 2 full grown whites (suggested by the pet store), and then I learned that I needed a way bigger terrarium, and ended up spending a lot of extra money. If you are shopping around for terrariums, you can punch the dimensions into
Cubic Inches to US Gallons (Liquid) conversion calculator
to try to figure out what will be best for you sometimes the ZooMed terrariums are cheaper than the Exo Terras, but the exo terras come in the handy kits. The zoo meds might come in kits too, I'm not sure.