It depends what you want out of your car. Because it has the same suspension with some minor spring rate changes to the E90/92 series you can use that info as well. I went with Koni Yellows so I could get adjustable rebound damping and I am happy I did as I went with Tein springs that are very close in spring rate to M3 springs so stiffer than the usual H&R or Eibach options, My ride is quite firm now and I wouldn't want it any firmer. Quite often you can get a combo with springs and shocks Bilstein especially and the spring rate and the damping are supposedly matched, i noticed a bit of pogoing in my E92 so with the Konis I could add more rebound to settle it down. I have also gone with M3 control arms which up front will give you stiffer bushings and more camber. Stiffing up the rear has a downside and that is no factory LSD so getting the power down can be tricky, perhaps less so with a no turbo car. On a budget shocks are your first upgrade and the more affordable options are Koni STRT or Bilstein b4. Next up Koni Yellows and Bistein B6/B8. Factory M Sport springs are good for daily driving, touring and some sport work but stiffer lower springs can help if you are really getting after it. They will lower your car but with good shocks ride comfort will still be OK. Getting rid of run-flats helps a lot for ride. A bigger rear sway-bar is good but again without an LSD it is a limited upgrade. Also depending on the vehicle your suspension bushes may be flogged out, at 70k my rears were getting pretty loose. and the M3 rear arms gave my rear noticeably less movement. M3 front arms give noticeably more front end grip.