Kings Cross to Heathrow by tube takes about an hour, I believe there are lifts at Kings Cross, so although you could save a few minutes by going to Paddington and taking the Heathrow Express, with the changing and the significantly higher cost its probably not worth it; I always take the tube from central london. You have to pay £5 for an oyster card, but you can cash it in when you get back if you are not going to use it again and you get your £5 deposit back along with any credit on the oyster, otherwise you can just buy a standard ticket. Look out for engineering works and delays and industrial action, you can check this out at tfl.gov.uk