I haven't heard wheels called mags before... just rims or dubs.
Maybe its because I'm in socal. From what TireRack says will fit the car, it seems that 13mm to 25mm seems to be an acceptable offset, with rim widths ranging from 7.5 to 8.5 inches. Whether getting 17's or 18's, a 235 width tire seems appropriate for this width range.
Not to hijack your thread, but I have a similar question. I am thinking about 18's because they don't cost that much more, the dimensions are still correct, and the weight penalty is not bad, because there is less heavy tire. I see aftermarket wheels made to fit E39 that are within the offset range, but I've heared hubcentric rings are needed. Is this also true with E60 wheels? If a wheel is advertised to fit the E34 and E39 (lets say the offset is 15 or 20mm) then how do I know if those rings are needed?