Why not go with the classic M-double spoke 18s? Those look the best on sevens, whether E32 or E38.
As for the Alpina 20s, I like them myself, but not as much on the E32 as the E38. You should be able to find several pics on Alpina enthusiast sites.
You should be able to keep the overall diameter of the tire close to stock. Those E32s came with 225/60/15s, for the most part, which are fairly large in overall diameter. Do the math and figure out what you can fit.