I'll give it a go, we've had chooks for about 3 years now;
I'd also start with 3. They're addictive, so I'd recommend buying a chicken coop bigger than you think you'll need.
As much as possible. It's not a helpful answer, but if they're kept in a coop and run, to protect the ground you'll have to move the run daily. If you can be sure you have a secure garden, they will happily run around the garden all day. Then, of course, their impact on the garden is minimal.
Very little; we have about 20 chooks and get through 2 25kg (£8ish) bags a month. We also buy chopped straw or paper for the coop.
Breed - I'd start with something robust to be honest - columbian blacktails are cute, light sussex are lovely, blackrocks are good layers. A speckeldy would also be fab. What colour eggs are you after?
We clean out once a week. With a smaller house, it's possible to get a pull out droppings tray. Then it's just a case of pulling it out and giving it a quick tap over a compost heap. That really keeps cleaning and smell down.
They are good at shouting when they've laid an egg. But we also have a cockerel which makes them more vocal (they like his attention!). Most of the time they just cluck quietly though.
Depends how close your neighbours are - if they're near then I'd offer eggs and hope they'll be keen enough to look after then when you go away!
Ours don't really smell at all. Although if you don't clean them out they will eventually honk.