Im a Certified BMW tech
The E30 bimmers are great cars, lil maintnance goes a long way! The M20 engine is great, one of the last truly great old engines. I'd be willing to bet it still has crosshatch pattern in the cylinders
If your getting a shimmy, and your rotors are good next usual suspect is tires/bent wheel or control arm bushings.
Is the shimmy/vibe just while driving? under braking? both? does it cease rapidly once the car slows a ways?
To check your front struts, need to jounce the suspension per corner, it should go down and come up and stop. It shouldnt keep cycling up/down. Should be dry, no oilyness. If they are bad, you can replace the strut cartrage inside, not the whole thing! Not too pricy. Need to check the whole susp for loose parts, use a pry bar to load/shift the joints around. The control arm bushings should not be cracked/torn/or soft when flexed.
If you have a bad ball joint, all of them are replaceable singley, need to be pressed in though.
Hope that helps!