I have four cats right now, 2 older ones (16 years) and two young ones (9 months).
Your £ 500 initial fund should be okay, depending on what you want. If you are dead set on a pure bred Ragdoll with papers, it could be tight, but if you don't want the papers, that takes a big chunk out of the price tag.
Our local RSPCA shelter has 42 kittens at the moment, including 6 pure Siamese cats. It's insane. I would definitely check with them, especially around June/July, as Spinner said. Around that time shelters usually have an abundance of kittens.
Where you get it from will also determine cost - if you get one from a breeder, even without papers, chances are that they will have their first vaccinations already, as well as being de-wormed and flea treated. So that will save you some money, even if the price for the kitten might be a bit higher.
My cats so far only had their initial vaccinations and never got any boosters. Probably an oversight on my part, but with them being pure house cats I don't see why I need to vaccinate them against something that they will never come in contact with. Also, some vaccinations don't need boosters. I also never de-wormed or de-flead mine after the treatments as kittens. They didn't need it. I guess that's different from cat to cat though.
I'm not somebody who spoils cats. Mine get what they need, and they get treats and whatnot, but I don't oeverdo it either. My kittens, for example, got kitten food for the first three or four months, after that I put them on regular cat food, and we had no problems. My old cats still get regular food - senior food has a tendency to make cats pack on the pounds. It's really only something for cats that have problems digesting the regular food with age. But that's not something you need to worry about for a long time!!
Oh, and my cats get whatever cat food is on offer, I think it is better not to get them used to just one particluar brand of food.
One thing I would do though is give the cat dry catfood. Even if you give them canned wet food, offer a small bowl with dry food as well. The reason for this is that older cats often suffer from problems with their teeth, usually caused by tartar, which might need to be removed by the vet (expensive) and which can even cause the cat to lose teeth.
On the last checkup (did a free one at the RSPCA), the vet said that by just looking at the teeth of my cats, he would have put them between 5 and 6 years of age. (As said, they're 16) They always got dry food on the side, and it keeps the tartar at a minimum and saves you a lot of money, and the cat a lot of pain in the long run.
I use the non-clumping litter from Tesco's, which is cheap but takes more work to clean (and needs cleaning more often). Other brands are slightly more expensive - if you go for the clumping kind, I would guess that a bag (for one cat) would last you about 2 weeks, and a bag of Catsan (which is one of the more expensive ones) is about £ 5 around here.
I'm also not a fan of the plastic bags that you can stick into the litter box - cats have a tendency to chew on plastic (or anything, really), and they will find a way to get to the bags and tear them. If a kitten swallows some of that plastic, it could be fatal.
And I agree on the two cats. Unless there is always somebody at home with the cat, it will feel better with a companion. Especially as a kitten. Mine were always together, curled up against each other, and even now they are inseperable.
Two cats don't really double the cost - buying them, obviously, and vet costs etc., but the litter box, scratch post and so on is pretty much the same regardless if you have one or two cats. But yeah, food costs double, so that is something to think about. But really....look at this and tell me it's not worth it....
talk you into it!