Rats are lovely pets, good choice 
You've obviously read up a bit already which is great. Cage-wise: as large as you can get away with basically. Some things are down to personal preferance though.
Aviaries make brilliant rat cages, they're big, tall so lots of climbing space, all metal so no chance of chewing to escape, generally cheaper than specific rodent cages and the bar spacing is usually consistent. However aviaries usually need fleece or another fabric instead of substrate as the bottom of the cage is usually bars. This can lead to more frequent changing of the fleece, slightly smellier cages and if you have rats that like to pull at fabric, rats under the lining of the cage which is fine but can be annoying. We used to buy binder clips in bulk to hold fleece down and the girls would still go underneath it.
Rodent cages are hit and miss but you just have to be a bit more careful with purchasing decisions. Many just aren't big enough, or the bar spacing is all wrong for young rats/females and there is a risk of plastic chewers chewing a hole through the cage. However they can hold substrate well and they are designed for rodents. The Ferplast Jenny is the bog standard rat cage that pet shops stock, it's expensive for what it is IMO but it is a decent size for a small mischief, well reviewed and easy to find. I've never had one but I'd heard that it's fine.
Savic do one called the Zeno 3 which is a good size and we've heard great things about it, planned to buy it ourselves tbh. We had a Coco Rat cage which was the same size but a bit shorter, so we were looking at a sideways switch really as our Coco was old, a bit broken and I was really struggling to clean it out as the metal frame just falls apart as soon as you lift it and as I was getting bigger (pregnant) I couldn't actually put the metal frame back onto the base without help. The coco is a fair sized cage though, especially for the relatively low cost, but has wide gaps at the sides, we had our smallest adult female (300g) in it for a while and she never escaped, but I think that's as much because she wasn't interested in trying to escape! I think she could have if she wanted too!
The Furet Tower is a popular cage, this was another one we seriously considered though our breeder had nothing good to say about it, apparently cleaning it out is a tad difficult, which is something we read from a lot of reviews and was a key reason why we didn't bother in the end.
One of the best cages available cages in the UK is the Savic Royal Suite, we upgraded to this in September and I do not regret it, but it's got a hefty price tag. At the moment zooplus is selling it for £200 which is a great price for it, we paid £240 for it from them, and you can get cashback from TCB on your first purchase at zooplus too. You do not want to try sourcing a second hand one unless you know what you're doing with refurbing a cage for rodents: we made that mistake this summer, got a second hand one and it was in a worse state than we or the owners could originally tell (they are people we trust and sold it to us for much less than they could have gotten for it so we trust them!) and, unfortunately, it needed sand blasting, something we don't have the tools to do and to do it would have put our expenditure on the cage up too high. So we caved and bought a brand new one.
It's piss easy to clean out, I'm 8.5 months pregnant and am able to strip it, scoop out the majority of the substrate, get the trays out, wipe the whole thing down and get the trays back in ready to redo the cage in less than 15 minutes per section. It'd be less if I didn't have the little sods running around my ankles as I did it 
It can hold a lot of rats, we have 3 boys in the top and 5 girls in the bottom at the moment. I will say that the substrate flies out easily though, we have to hoover around the cage most days but it doesn't take long. I have been told that B&Q do cement trays that fit exactly for under £20 which have higher sides but I haven't bothered as our Henry sits a metre from the cage, so I just plug it in and quickly sort it. It is an investment cage though so you need to know that you really want rats or other large rodents in future! You can sell them on for a good amount when you're done, not many come up second hand and when they do they're often gone within hours of being posted, I've heard of people travelling from Brighton to the Midlands to pick these things up before.
We've also owned a Fiesty Ferret, which we did have a baby rat in, the bar spacing was possibly slightly too wide but again, she never bothered trying to get out (our smallest girl actually!). It was a pig to clean out as you had to lift the trays up and onto their sides to get them out of the door, only one side of the front actually opened. It was wired on the bottom so needed fleece, but using binder clips to keep it down was virtually impossible as they'd get the whole tray stuck, but without them the rats pulled it up in minutes. We switched to vetbed when using it but even that was a pain as food gets caught in it.
We've had this before too which I can highly recommend, the hamberley aviary, however it's a tall cage and I would have old rats in it as I'd be worried about them falling! We bought a second one as we liked it so much though and did plan to take out the mesh on one side of each and put them together, though plans had to change unfortunately! I'd seriously consider doing that again had we not got the SRS tbh!
We have had plastic chewers in the past which is why I have gone through a few different all-metal cages!
You'll also want to think about getting a second, smaller and preferably single level cage as a hospital cage, just in case you ever have one that breaks a bone or injures themselves in another manner. Breaks and sprains heal very fast, but they are prone to reinjuring themselves by continuing to climb around despite their injury because they're stupid like that sometimes so if you have a particularly tall cage especially, you'll want something shorter to stop them doing that. We have loads of spare cages for all sorts of reasons. We have a tiny cage that was bought as an emergency hospital cage back when one of our first pair had a freak accident, but now gets used a lot as a short-term cage as it's slightly bigger than a carrier and has a flat bottom, so it just fits our five girls in together. It gets used for short periods like going on a drive, going to the vets, keeping them contained if I'm cleaning the cage and they're being particularly mischievous, etc. It's tiny so not at all suitable for more than that, but I actually use it very regularly! I have a Mamble 80 which is a great holiday cage, the Mamble 100 is a better size though admittedly, but it's fine for a week for two or three rats if you are taking them to somebody else to look after. The Grosvenor is the biggest single level cage you can reasonably get a hold of, or it was when we needed one for a special needs boy who couldn't have a higher cage.
A note about the Little Pet Warehouse: It has a lot of things on it that it claims are suitable for rats, do not buy anything from them without checking that they actually are suitable though! A lot of their wheels spring to mind but some other bits and pieces on there are dubious, they're really good otherwise, very cheap, deliver very promptly, they just have low standards for suitability!