Yes you would need associated aircon components, and yes it is a major major job, major enough that you'd be better off buying one with climate control already fitted.
The heater box is all different because it's servo controlled, there's cabin sensors, different wiring looms etc, the whole dash would have to come out and you're talking lots of hours, and you'd have to be pretty clued up on electronics and the vehicle in general to do that.
I've never done this on a Discovery, but I have done it on a Vectra, and started to do it on a Mondeo and it is big, big work best left alone and buy a model with it fitted from factory in my honest opinion.