Bilstein B12 struts and Eibach lowering springs is a good start. I would also do Dinan upper camber plates too. Replace the rear shock mounts "while you are in there" they will be whipped. Also replacing the sway bar dog bones(or links) is also a good idea. Upgrading the suspension is going down a rabbit hole.
I would also say having some sticky rubber is a good thing too. Then do the M3 control arms front and rear and new HD tie rods. And then sway bars.
Also don't forget to replace the rear suspension bushings to the M3 ones. That really firms up the ride and transitions - when changing direction mid corner.
Wheel spacers(5mm to 10mm's) are also good for looks and a slight performance upgrade.
You are probably gonna want to upgrade your brake rotors and pads too. And brake fluid.
Some good threads for reference...
***The ultimate handing machine: 128i ?
https://www.1addicts.com/forums/showthread.php?t=575027
**TC Kline Racing - Performance Suspension 101 (PS101)
https://www.1addicts.com/forums/showthread.php?t=982503
M3 front control arms (Upper and lower) - A couple of $hundred less than everywhere
https://www.1addicts.com/forums/showthread.php?t=736789
128i: Simple Fun, Great Price! ...in Bimmer Magazine...
https://www.1addicts.com/forums/showthread.php?t=734600
Dackel's replacement of rear suspension OE bushings with M3 ones…
https://www.1addicts.com/forums/showthread.php?t=638163
New Suspension Installed w/ Quaife LSD - DIY
https://www.1addicts.com/forums/showthread.php?t=513534