I'm a breeder, so you probably don't want my comments. There are actually a ton of us here, and I hope you'll consider any wisdom we have to pass along. We've seen it all.

Frozen vs. live. Honestly? Whatever works better for your family. If you decide to feed live, you'll either then have to keep and breed rodents in the house (something the missus might not like) or buy from your local pet shop, often at high prices when feeding time comes. With frozen rats, it's very easy to have a small supply of rodents tucked away in the freezer. You can keep them triple wrapped in plastic, or partitioned off by cardboard from the rest of your food. Thawing is easy.

Separate tank. Completely unnecessary.

Male vs. female. Completely up to you. Females can get bigger, but there are some pretty hefty males out there, too. One is indistinguishable from the other temperament-wise.

My advice would be to get your first animal from a breeder. Maybe you'll be lucky enough to have one nearby that you can go visit. They'll back up the health of your new pet, something you might not get from craigslist. As far as value and morphs, the only limit is your wallet. Get your daughter the animal she falls in love with, and that should keep her interested for years to come. More likely, it will spark a "fanatical" love for these animals, and plans to add more of them in the future.

Good luck!