Ooo, I'm a rat fanatic, I have one in my shirt at the moment :)
Best to get them in a pair or trio, I'd recommend a trio just in case something happens to one of them. We started with a pair of baby boys in January, adopted another pair (female, year old) in March and another pair of baby girls last month because one of our girls was dying so we needed to have some company for the female who was not.
The best place to get rats is from an NFRS breeder (National Fancy Rat Society), where abouts in the country are you? I bought my babies from two different breeders, my boys from a show breeder and my girls from a family breeder, I'd recommend both but the second pair of babies were so so much more sociable from the start. You can expect to travel for a decent breeder though. If you're South East let me know :)
You would need a big cage, I can wholeheartedly recommend this cage:
www.littlepetwarehouse.co.uk/hamberley-single-metal-large-cage-p-12607.html
I have my girls in this cage, held four quite easily :) The cage spacing is perfect for babies and girls, though be careful on that site as they say a lot of things are suitable for rats which aren't. All metal cages have the advantage of not being escapable, our rescue girls chewed through two plastic cage bottoms, our boys aren't chewers though.
They need quite a bit of time out, at least an hour a day, but a lot of that can just be shoulder time, we rack up an hour in shoulder time for our five easily enough.
Don't be fooled into thinking they're all lap rats though, girls are more likely to be active whilst boys are more likely to be docile, but it's all down to personality. One of our boys loves to sit on our shoulder, but he also loves running around behind and under the sofa! Once they're a bit older and less active you can free range them in a rat proof room.
They're lovely creatures, I love my mischief, but they have their pros and cons. You do need to research them before committing to buy them. They're susceptible to respiratory infections so I'd recommend listening to some sound clips of how rats may sound when ill. Once you get to know the signs you'll be fine though. Their claws can get bloody sharp too, but there are ways to control that and you can clip them but you should look up instructions to do that. I clipped mine for the first time last week and they survived ;) We kept our boys nails in control with rocks and slate in their cage for months, but they did like peeing on them! We've yet to replace them and have the scratches to prove it!