I am going to suggest a different approach. It is not a huge space so this should not run massively expensive. I would frame it up normally, Insulate it with either fibreglass or extruded foam insulation (blue or pink). The interior is likely to be warm and humid, Drywall tends to respond poorly to these conditions so I would avoid it. I personally would use something like:

http://www.multipaneluk.com/products...alupanel-lite/

It is resistant to water and pre-finished once it is up you are basically done. It will not remove humidity from the air the same way drywall will and is more durable. It costs more but saves a lot of time, and does not need to be painted, so in the over all scheme it may be about the same price. Absolutely if you have to hire drywallers.