If you can avoid it, don't get rats from a pet shop. They usually come from Rodent farms which are utterly horrendous places. A good breeder is your best bet, you can find a list of breeders accredited by the National Fancy Rat Society on their website, just Google "NFRS breeders list" and it should take you straight to it :) you may have to be willing to be added to a waiting list and to travel a short while, usually breeders waiting lists aren't too long, they are also longer if you want specifics so if you're happy with either two boys or two girls of any variety you'll likely get rats sooner, whereas if you want dumbos or a specific colour you may have to wait a while depending on what they are breeding. I'd recommend just going for two generally, you can always get specifics later!
Your daughter has obviously done her research if she wants a pair,i would suggest considering a trio. Rats can have accidents or illnesses that may leave you with just one unexpectedly, and they get very lonely when alone, so you don't want to be in a situation of needing to find another rat quickly to replace lost rat. We have a situation when our first boys were small and one needed an op to remove his eye, we were scrabbling around contacting breeders for a few days when we weren't sure if he'd make it. Ironically he did make it but nearly two years later his brother needed an eye removed and didn't, we had the one left alone for 4/5 months and he was miserable, finally found a companion for him in March and now they have two more companions, but it would have made life much easier had we form for three originally!
What cage are you looking at? Get the biggest you can afford, bigger is always better with rats! If you're looking for hammocks this woman makes brilliant ones, they've withstood our girls attempts to destroy them more than others we've bought: bettysbeds.com
With some initial investment you can sort food really cheaply by making your own mixes, much better than shop mixes and cheaper on a monthly basis, but you need to buy a lot of ingredients at the start to make it up, usually costs me £120ish every 6 or so months to make enough to feed 7 rats in that time, much much cheaper than standard mixes! I have 9 now and they are currently eating ridiculous amounts, I'm interested to see how long my next batch will last!
Also keep in mind to put money aside for the vet each month, rats are prone to respiratory issues and to tumours, some may go without ever needing to see a vet or going a couple of times in their life, some will need semi-regular treatments. Check out your local vets, make sure that they have small animal specialists or, better still, a rodentologist, and call to check how much they charge for routine appointments, a prescription of something like baytril or metacam or veraflox (common meds for rats to need, the middle is a pain killer and the other two are antibiotics) and for tumour removals or other ops. Better to know before you need to use those services!