with the rims I would not go over 17", 18" I tried but as a daily driver a bit hard.
Lowering spings
BMW 7 Series (E32) lower springs, 7 Series (E32) lowering springs, 7 Series (E32) Coilover Springs Free Shipping
I would go for H&R
H&R Sport Springs For BMW 750iL 1988-1994 (29666)
55mm front and 35 mm rear lower I would suggest, if you want it.
Of course you could make it extreme, but...