You'll need a few hammocks, we change ours midweek as they like to pee on them. But then we also use old t-shirts with safety pins in as hammocks!
Make sure that you have a few baggy jumpers that you don't mind being wrecked. Our boys started out by being chucked in a zippered jacket, now when out they will dive underneath it. And into my bra 
RE illnesses, in the past month we spent £25-30 on getting one checked out for a respiratory infection and ABs for it, £100 on another who had his eye poked out when play fighting and I think that we're going to have to convince the vets to give us some more ABs as three of them have definitely been sneezing, but whether that's changing the substrate or not I don't know
We'll wait until next week. They can be expensive little buggers.
If you get females you'll need to seriously consider having them spayed quite young as they are prone to tumours, which spaying will radically reduce. We've been quoted from £40 to £100 per rat for this though our girls are rescues that we got at a year old and the vet has said that it wouldn't be worth it for them because of their age now, it would only partially cut the chances of tumours now but they'l still have over 30% of getting them. Had they been done younger it'd be less than 10%.
Don't let your DD get discouraged if they are shy either, they could be really friendly immediately, or it may take a few weeks if not longer for them to be happy with being picked up.
I've only had rats for a couple of months but I'm obsessed, love them! DP is unimpressed by my notions to get more though :(