Hi there! Welcome aboard.
And BLESS you for doing research before you bring a rat home. So many don't and have to muddle through or have disappointing pet experiences. We'll all be happy to help you learn and give you info to decide on what works for you.
Cairnes...that's Queensland, Australia right?
I'm southeast US, but I'll see what I can look up for you?
There's an active rat fancy community in Australia. You are in luck because many of the illnesses and health issues we deal with in the States are not a problem with Australian fancy rats due to the strict non native animal policies and QTs they keep in place.
The Dapper Rat site is devoted to rats based in Australia. They've got great tips, and lots of rat care advice, including cage suggestions and care. There's also a link to rat breeders in your area, though I'm not sure who's currently breeding or how up to date that list is.
Don't be shy about contacting breeders and rat owners directly and start asking them questions just like you are here. Rat ownership is kind of a tight knit grassroots thing on the web, and the best place to get accurate advice on rat care and support as long as you're willing to do a little foot work.
To answer some of your questions quickly.
Rats are outstanding pets in my opinion for the right owner. They're smart, people bondable, and tend to be cleaner, gentler, and more entertaining than most other rodents. You will need to keep more than one to keep them mentally balanced and happy. Keep them in same sex pairs or trios. With proper introductions, males can be kept with males, and females can be kept with females each with no problems. Unlike, say male mice, who must be kept solitary or they will kill each other.
Males and females have different personalities. Females are active, silly, and tend not to sit still. Females may be a bit more responsive to trick training and play. Males tend to become lazy and fat as they age, and are the ones who often become sleepy shoulder or lap rats. There are other sex related differences, but that's good for a start.
Healthwise, while rats give you more bang for your buck as pets-I compare them to cats/dogs in a tiny package-for their size, they tend to be more expensive to keep as a pet. They are prone to tumors and resperatory issues. Some of which can be prevented with spaying, proper cage set up and bedding, regular cage cleaning, balanced diet, and breeding for low tumors and a strong immune system (health). This sounds like a lot, but when you break it down it's not so difficult. Think of keeping a rat as like keeping a dog without a lot of the dog hassle such as having to take it for walks or listening to it bark at all hours.
Cage needs: Rats are intelligent and love climbing, but have weak respiratory systems. You will need a wire cage to prevent ammonia build up that irritates lungs. Avoid any glass tank type caging. The cage will do best if you can provide lots of climbing surfaces and levels. The bigger, and more numerous the better. An individual rat will require at bare minimum (with lots of outside cage time) 2 cubic feet of cage space per rat. It will need to be no less than 18 inches deep at it's narrowest point. The wire spacing should be no wider than 1/2 inch for females and juvenile rats (they can escape anything they can fit their skull through), though big male rats can be kept in cages with 1 inch spacing without fear of escape. If the cage offers powder coating it's worth the expense as galvanized steel alone absorbs urine smells. They will need at least some solid floor space as all wire flooring will promote foot problems. The bedding should be low dust and odor and toxins like ink, with low risk of tangling toes in cloth fibers. Bedding to never use (as nibbler said): Ceder or Pine bedding which contains high levels of phenols that cause respiratory irritation and can promote other avoidable dangerous health problems as well. High dust or perfumed beddings. Beddings that are safe: Felt or Flannel liners-usually hand made from remnants from a craft store, or Aspen wood shavings.
As an age, 5 years old in general tends to be a bit young to have a rodent pet because they are so small and fragile, but it all boils down to the parent and the child--how much the parent is willing to be in charge of vet, daily care, and supervision for the safety of the animal and child, and how gentle the child is around animals. You know your situation, so you are the best judge for if rats are going to be right for your sweet daughter.
As Nibbler said, rats are incredibly social, so you'll want to get at least 2 rats together. They really do need to be kept in pairs or trios as pets. A solitary rat will invariably be a neurotic, depressed, and possibly more nippy rat, even with lots of human interaction. Unless we can offer 12 hours of constant human companionship, we aren't going to be enough by ourselves.
The lady who was breeding them for her snakes sounds like she was probably practicing poor animal husbandry. Rats are excellent mothers, so if a rat is killing her babies like this, it could be because of any of the following: she is being bred too young, too much, in stressed conditions, or even being poorly bred with serious health or abnormal temperament problems. Any of these could give you babies that end up with excessive health or temperament issues once you bring them home.
Feel free to check out my blog on this site. I've got several entries on obtaining and caring for rats from a newbie perspective. My rats are my first rats, and I'm a compulsive researcher, so I figured I'd share what I've learned in the last two years.
Links for you:
The Dapper Rat Aussie Site: http://www.dapper.com.au/index.htm
Aussie Rat Breeders, info forums etc: http://www.dapper.com.au/links.htm
Nibbler already provided the cage dimension calculator site for you so I won't include that again.
Some of my blog pages:
Selecting a Rat For You: http://www.paw-talk.net/forums/blogs/storyseeker/239-selecting-rat-you.html
Where to Get Your Rat: http://www.paw-talk.net/forums/blogs/storyseeker/238-where-get-your-rat.html
Expected Cost of Keeping A Rat: http://www.paw-talk.net/forums/blogs/storyseeker/3936-expected-cost-keeping-rat.html
Links! Rat Cages and Accessories: http://www.paw-talk.net/forums/blogs/storyseeker/3943-links-rat-cages-accessories.html
Bringing Home Baby: http://www.paw-talk.net/forums/blogs/storyseeker/3941-bringing-home-baby.html
Rat Proofing Your House: http://www.paw-talk.net/forums/blogs/storyseeker/3939-free-range-rat-proofing-your-house.html
Best of Luck to you!