I've done long hauls with my son (longest trip at 13 months, 30 hours in the air, plus layovers), but never solo.
It's hard work, even with another person.
A few questions: are you buying a separate seat or will the child sit in your lap? Are you nursing or bottle-feeding? What sorts of solid foods does your child eat? Can the child walk yet?
Airplanes have changing tables in the bathrooms--but not in ALL the bathrooms. Ask the flight attendant when you get on board which bathrooms you can use to change diapers.
Flyertalk's Travel with Children forums have a lot of good suggestions for long haul flights:
http://www.flyertalk.com/forum/travel-children-221/