Hi Queen, your son has good taste!
You'll need a big cage for starters. The Ferplast Jenny is a common one, but aviaries are also brilliant cages. I have my two groups in Hamberley aviaries here we bought this initially for our girls as they were big plastic chewers and so plastic bottomed cages became unsuitable for them, but got one for the boys eventually too. Height is good for rats, they love to climb and climb well.
Next you'll need accessories, rats generally love hammocks and your cage will be full of them. We don't have shelves anymore, just hammocks. Ours range from lovely handmade hammocks like these (though we get most of ours from our breeder these days as she knows somebody who makes them and the profits go to her rodent rescue) to just old cut up t-shirts. We have a lot of fleece too, ikea sell a fleece blanket for £1.50ish which we stock up on every few months but we also use fleece on the bottom of the hamberley cages too instead of substrate. You need to cut big drops using hammocks or shelves really as they can get injured. We have a girl who is recovering from a broken wrist, we're assuming she fell. Last year she managed to sprain a wrist, break both and then break her hip within 6 weeks, mostly fluke! The hip I really have no clue what she did given that she was in isolation with me at the time!
Toys are great, if you want a wheel you need to go for one that is big, the silent spinner 12inch is quite good, but pricey. Ours use the wheels as glorified toilets tbh though, totally not worth it. We buy wood toys occasionally and wooden bird toys are great for rats. We make a lot of their toys though. Loo rolls get stuffed with treats, small boxes get stuffed with treats and closed, aboxes generally are thrown in for them to do with them what they will (usually tearing them apart and/or nesting in them!)
If you have a plastic tray type cage you'll need substrate, we use paper pellets for our loner's cage, some wood shavings can be okay (in the UK at least, it's to do with the wood used to make the shavings as the oils and phenols can be damaging to a rats respiratory tract) but a lot aren't so we avoided it until our boys became allergic to every other major substrate available including carefresh. Ratrations has a lot of substrate options that are similarly priced to shavings though. They do food too and are a good site for most things!
Foodwise you need to look into really. Most people I know follow the Shunamite and either make up their own or buy it from people who make it up. Just pellets aren't very interesting for rats but they can be fussy with muesli. I make my own mix and it's catered to my rats, over time I've worked out what they will and won't eat, I add things specific to their needs (older female, boys with constant URI problems, boy with epilepsy, etc) and it costs me around £60-100 every 5ish months I think using ratrations and buying cereals from lidl and asda. So not bad for 7 rats!
A large hamster cage is good for a hospital cage and/or travel cage. We have tonnes of the things now!
Finding a vet who has rodent experience or is a rodentologist now is good. We're lucky enough to have a rodentologist and she's fantastic. We've had to see her a lot!
Find a good breeder. The NFRS has a list of them. You may need to travel to get rats, we travelled 2/3 hours to get our boys (well, DP did! there were some closer but some weren't breeding at the time and others had waiting lists for their waiting lists!) but we got lucky the next time we got babies as we found an amazing breeder on our doorstep, if you're near Brighton I can wholeheartedly recommend her! Here is the FB group of my local breeder. It's worth joining as there are a lot of people with a lot of good knowledge all over the country on there.
I wouldn't recommend getting rescues as your first pair/trio. You don't know their history and they can be very difficult. They are very rewarding and my most rewarding rat was a rescue but it was hard work. I have 5 rats who I got as babies and have 2 rescues currently but had another too (the very rewarding one, she died last August from a tumour).
Look up the sounds of rats with Upper Respiratory Infections and read the symptoms lists. It's the most common illness in rats and whilst it can be mild and not need treatment, it's good to know when you need to seek treatment as it can kill them if it gets to the lungs and they can deteriorate fast. Another thing to know is that rats can get something called porphyrin. It can be caused by stress, irritation to their respiratory tract, URIs, etc. It can come from their nose or their eyes and it can look like blood as it's bright red. Do not panic if you see it, a lot of people do and it's perfectly normal! Though only once have I seen one of my rats get it from their eyes and it was a little weird!
Tumours are also common in rats, especially females. Usually they're benign and many rats live perfectly fine with them, but it's worth checking with vets how much an op to remove them would be in the event that you get one that needs removing. Spaying a female decreases the risk of them getting tumours if done before they are 12 weeks, but it's a choice you need to weigh up.
I'd consider getting three rats to start with. If there is an emergency or an accident you don't want to be left scrabbling around looking for a rat to join the rat left with little notice. It's not likely to happen but it can happen. Our Howl had to have his eye out at a young age, no more than 15 weeks IIRC, his brother poked it when they were play fighting and caused it to rupture. We honestly thought that he wouldn't survive the op. A year on he's fine thankfully! Had a few accidents since but nothing too major
There are differences between males and females usually. Bucks (males) tend to be lazier and more cuddly. Does (females) more energetic and hyper. But whilst I find my boys lazier, my two big bucks don't like cuddles. At all. They get forced into them for short periods but would rather explore. Our girls are more hyper but they will happily sit in a shirt or bra for a good hour having a cuddle. I have a runt boy as well who is very cuddly, but that's more because he's a runt than because he's male.
If you aren't fussy about colouring or varieties then you'll likely be able to skip the waiting lists a bit, as many rat owners have types that they are after. Like I know that my next baby will be a female silverfawn preferably and so I'm waiting for the second litter of them from my breeder (the first is due this week so it's a few months from now as DP has vetoed anymore for now!). Girls also seem to be more available down my way too, I think people prefer boys generally speaking.
The photo is of Ashi, my runt. He's my heart rat, absolutely love him!
Sorry for the essay, let me know if you need to know anything else!