Hm... I hope you're not too set on that gender. Babies can be tricky to sex even to the most experienced herpers. With the cage, I wouldn't recommend something that big for awhile for a few different reasons. A hatchling can get stressed in a big cage, and stressed herps are less likely to eat. Large cages make it hard for tiny hatchlings to find hides, water, ect., also causing stress. Depending on what substrate you choose, it can be darn near impossible to find a hatchling in a smaller enclosure, let alone a big one - it's like finding a needle in a haystack!
I'd recommend using a ten gallon, large/XL Critter Keeper, or rubbermaid shoebox untill the little guy gets a bit bigger.
And just a tip - ask to see your baby eat before you take her home (or maybe a day or two before so her stomach can settle a bit after the meal before you move her) and ask how many meals she's been offered/taken and what they were. The more info you can get, the better - good breeders keep logs on almost everything they do with their animals. If you can get a copy of the record for the baby you take home, do it - this info can be extremely helpful in getting hatchlings to eat.
It's funnier in Enochian.