-Hens in paved courtyard garden: It's not the perfect option because they like to scratch in the earth and hide in foliage but it's perfectly possible and many people have happy chickens in such circumstances. You would make a wood chip base for the run and replace the woodchips from time to time. If you let them free range in the rest of the garden you would have to be prepared to do a lot of poo scraping. If you are urban there are likely to be foxes and you might want to think about the biggest possible foxproof run.
-Start up costs: few hundred total, depending on how much you spend on the coop & run, which will be your main cost. Probably £100 + cost of coop and run. For the coop and run you could do anything from buy the fanciest possible for £800 to converting an old playhouse or shed for a few £.
-Running costs: not great. Few pence per day on feed for each bird, ten to twenty pounds or so a month on other stuff. The thing which will make it go up is if your birds get sick and you are a kind and loving poultry owner who takes them to the vet a lot, rather than a ruthless one who culls them.
-Chicken poo - Wanna see some pics? If my calculations are correct I think the quantity is something like 2.5% of a cubic metre per bird per week. Call it a few shovelfuls every week, anyway.
-Brown bin - ours can, I suppose it depends what the rules are round you. I compost mine and you might find someone who wants it for their allotment.
-I don't think cats are a problem either way but we don't have one so not sure.
-You won't be getting a cockerel so noise should be limited to a bit of noisy clucking when they lay an egg. I have a sense that in most cases where the neighbours have complained about noise from a few backyard chickens, there's another underlying issue and the neighbours are looking for something to complain about. I never even noticed my neighbours had hens. People are generally very positive, especially if you give them eggs from time to time.