I'm sort of in a similar situation as you are. Though I'm not one to do mods for looks (I think the Z looks fine from the factory) but I do want to make it as reliable as I can and to forestall any future problems by learning from other owners past experiences.
Not a mod as such but I ordered, and now have on hand, gear oil and trans oil for the differential and the manual transmission. That I will also do once the weather is nice and I have made my lifting bars so I can jack both sides of the car up evenly to properly do the change.
OK so one thing I did change that is somewhat on the looks end is a stainless steel gear shift knob from Storm Motorwerks http://storm-motorwerks.com/interior/bmw/shift_knobs_v2/. It isn't totally a "looks" item as the shifting seems much smoother what with the shorter throw and heavier gear shift knob.
If it doesn't run well...who cares how it looks...Just my thoughts and plans.