I don't know the spec off hand but the primary side should be less than 1ohm. Testing on the 20k scale is useless.
The proper way to test a coil is dynamically with an oscilliscope. Looking for available voltage and proper oscillations.
I seriously doubt that your rough idle is caused by the coil, but I suppose anything is possible. Idling is one of the easiest situations for the coil. Starting and acceleration tax a coil much more.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician