There are actually tons of discussions on springs and shocks for the e34....
I would run either the eibachs or H&R's with good confidence. When I do, it will be with the Bilstein comforts already on the car.
Lowering the car lowers the center of gravity. Stiffer springs/firmer shocks, limit body movement enhancing control. These things also contribute to keeping the tires more squarely planted on the road enhancing traction. Those are the basics.
The different combinations result in all kinds of compromises from ride quality to ride hieght to handling balance. As mentioned there is tons of annecdotal discussion on the boards about what works.
Current ride... '13 GTI Wolfsburg
BMW's from my past...
'92 535i/5 '89 325ix/5
'98 750iL '86 325(e)/5
'95 740iL '84 318i/5