Rats are amazing, all you need to know 
Cagewise you want the biggest you can afford. Aviaries make great cages for young rats, but they need loads of hammocks to break falls and you'd need to budget for a cage with more floorspace once your rats get older. The ones people seem to buy often (though they're pricey) are the furet tower and the Savic Royal Suite. We have the SRS, bought it just under a year ago and I adore it, we're now using half of it as our mischief has sadly dwindled and we're down to 3 in it at the moment, but it's still wonderful. We upgraded to it whilst I was heavily pregnant and it was a lifesaver. We seriously considered the Tower but a common 'issue' on the reviews was that the doors were a bit too small to comfortably hang hammocks, my breeder didn't rate them highly either which sealed the deal for me. We've had aviaries and other cages in the past though.
They need a lot of time out of the cage, they're really intelligent, ideally an hour a day. They can be trained quite easily but it takes repetitiveness and commitment, we had two very clever girls who we trained occasionally and, until they both died in the past 6 weeks, they could do a few tricks without being 'retaught', but the rest of our mischief are a bit stupid and need more regular training to keep up and we often get lax, particularly since DD was born as free range time is often interrupted by the baby!
We let ours free range in the evenings for about 45mins to an hour or so once DD is in bed. They occasionally come out during the day too and we give them cuddles during the day to get them used to DD and her used to them.
I'd recommend 3 to start with rather than a pair, we had a freak accdient with our first pair where one poked the other one's eye out when they were three months old, we thought we'd need to rush out an find another rat to keep the boy company but thankfully our one eyed wonder survived the op to remove the eye and lived until he was 3. We did get more soon after that though.
Look up how to give them a good diet, there's a big divide on what's best, on American based forums you'll often see about lab blocks, they generally feel that muesli style mixes are inferior whereas British forums tend to lean towards the opposite view, which makes things really helpful 
We make our own mix by buying the ingredients in bulk from ratrations and basing it on the revised Shunamite diet which is found in The Scuttling Gourmet book, not the one that you often find around the internet annoyingly. This page has a good guide to a good mix if you fancy going that way though:
www.isamurats.co.uk/options-for-dry-mixes.html
That website has alot of great info on it as well.
Females are prone to mammary tumours. They're often benign and the risk can be lessened by having the rats spayed at a young age but it's a very personal decision, we had our first two young females spayed but not the next two and likely won't get our next three in October/November done.
Rats generally are prone to respiratory infections, it's worth listening to some sound bites of what rats sound like with mild infections and more serious problems and also other symptoms (like porphyrin, which looks like blood coming from their eyes and nose but is entirely normal and not blood!) Rats should be silent but mild URIs often spring up from environment changes and stress and usually resolve themselves within a few days, but if they don't seem to be getting better or they start getting worse you want to see a vet ASAP. You get more confident in judging it as you get used to rats, we have one who gets snuffly at the slightest atmospheric change, and then does squeaky snores as a result for a few days, she's fine though and has never needed medication.
You'll want to ring around vets and find out what they're experience of rodents is as usual vets are very hit and miss with rodents, we've had some give dangerous advice in the past and others who have been wonderful. We were spoilt with a rodentologist for the first 18 months of rat ownership and she was wonderful, I was gutted when she left our practice!
Charity shops are great for buying bits and pieces for rats. We've picked up wooden wine racks for 50p which are great cage climbing toys. Old clothes and pillowcases get cut up for tubes and hammocks, kids clothes can go in whole if they're small enough.
Parrot toys make great rat toys and you can find tutorials for making toys from stuff at home easily enough too.