You don't need any sort of a fan for the Aga itself.
If you're going to cook on the hotplates then you probably need an extractor to take away the cooking smells, just the same as any cooker.
Contact Cambell&Cooke and they will give you an explanation. We cook on an AGA and don't an extractor fan - as per C&C. Hasn't casued any problems - even with smells.
For oven cooking you will need a stopwatch as you won't smell the food cooking / burning so need to watch the clock more than you might with a regular oven