Any disabilities must be declared at the time of booking, although it is believed than some people don’t declare it then as they are worried about being refused the booking. Facilities at airports and on-board are unfortunately still limited, so as much notice as possible is required.
The hard part is finding out which company is responsible for looking after you. The airport may assist you to the check-in desk, and then the airline takes over, for example.
When informing the airline (usually by calling a special number or their general call centre) always take a name as reference in case of problems. If the promised assistance is not there for you, the airline will just refuse to believe that you booked it unless you have a reference of some kind.
Powered buggies may be allowed in the hold, but there are requirements, such as the battery must be disconnected, before they can be loaded. Wheelchairs are also placed in the hold; with passengers using airport or airline ones after check-in.
There may be extra charges for equipment that needs to be carried on such as oxygen cylinders.
Some airlines are particularity unhelpful when you’re looking for specific requirements, so instead of carrying out general searches consider using a specialist tour operator, or contact forums to find out how others have fared when flying with a particular airline.