Stop thinking about this, it's not possible. I'd tried it last summer but when had the whole dash removed I saw the intake of the engine should be removed too. I have a complete description (original BMWs airco built in manual) but it's really impossible (I had not all parts but I started with it though
). The better way is to buy a car the airco option.
BTW In the Netherlands a qualified company should build in these airco installations, otherwise your car wouldn't pass the yearly safety inspections.