BMW E-series has several steering rack options and ratios:

• hydraulic power steering (heaviest) - 6cyl engines, etc
• hydraulic power steering with servotronic valve (lighter) - M3 rack
• electric power steering (EPS) (lightest) - smaller engines

Hydraulic power steering can be recognized by the steering fluid reservoir cap under the hood.

(For best lateral performance, OP may need manual tuning to account different assist levels).

It is unlikely that E-series EPS steering can be controlled via CAN bus (though someone could try to hack into it e.g. like this ).

While the steering racks differ, the steering column for E8x, E9x are mostly dimensionally the same (so far).

Type A (corrugated)

Type B (ribs)

(They are also very cheap.)

Steering column shape allow for the actuator to be attached on its retractable shaft:

So far there are two solution:

• Nema17 geared motor with helical gears
  • good for most roads
  • uses two half-gears that click on the shaft
  • doesn't feel smooth at high load
• Nema23 motor with a belt and pulleys
  • it's a work in progress
  • should be good for all roads
  • needs removal of shaft end-bolt to install the belt and the pulley
  • should be smoother under load