i love mine as it is, got a stubby and led number plate lamps, cant see me going much from factory spec, I'm talking to the dealer where the car originally came from to get the number plates and tax disc etc all back to how they was when it left the dealership, I'm a bit of a perfectionist with those silly side of things, (call me mad)
for me, not much modifications, original Z4 + spacers + lowered (20 mn)
and for sound exhaust, i'd go for F1 type, loud in the grave and metallic sound in the treble ..
http://www.youtube.com/watch?v=iR6edKfPUTk