It's a matter of taste of course. I would go for the M-kit. It's relatively discrete, blends perfectly in with the general design, in the same time it gives the car a slightly more aggressive look.
If your car has got the adaptive chassis, that's all you need, unless you want to lower the car. If it has the standard chassis I would consider installing a sport suspension. To me it would depend of my driving scheme. If close to 100% urban traffic I would keep the confortable standard chassis. If you go for a spin outside city or even take longer journeys from time to time, I would seriously miss a sporty setup if I were you (unless you've already got the Adaptive Drive option).
The car is fast, but not brutally fast as the 650. Allthough acceleration figures are not that different. I guess the difference in torque is more important. Before you consider an ecu upgrade, get familiar with the car as it is.
I don't miss any extra power in my 640i, but that's just me!