For me a premium car like BMW Mercedes etc. has to have all the toys. Makes the car nicer to live with and much easier to move on at sale time.
At a minimum iDrive, sports seats, heated seats, folding door mirrors, cruise control, PDC (front and rear parking sensors), wind deflector would be nice but if the car doesn't have it, it can easily be retrofitted.
I am on the fence about the M Sport model being a must, whilst I think the front bumper looks great the back bumper not so much and not too keen on the all black steering wheel or the carbon alu interior trim that came standard on M Sport cars. M Sport has a harsher ride without actually handling any better than the standard SE model. Oddly the Z4 is one of the few BMW models where I don't think M Sport really adds that much.