Pretty sure Recaro makes bases for the Z4, as well as VAC motorsports
http://www.vacmotorsports.com/ .
The airbag light will be on, I don't know if you can get the dealership to turn it off or not.
And if you were in the US, I'd gladly take the M seats off your hands. Good condition, common colored leather would probably go in the $500-1000/pair range.