BMW 3 Series On Board Computer - Sparrow
I am thinking of getting a BMW 3 series diesel, the lower specc'd ES model is quite a bit cheaper than the SE. It has some extra "sporty" features but misses out on things like cruise control, parking distance alert and the On Board Computer (trip computer). The first two can be added from the options list, but BMW don't seem to list the Computer (it gives miles to go, average speed, average fule consumption etc) as an option.
Does anyone know if its possible to fit it after the car is bought. Do I just need the stalk with the button on it and if so how difficult is it to fit?
Or have BMW made it more compex than that?

Many thanks.