I thought that Spartenburg (the only plant to make Z3s in production) started out with a water based paint shop. The article here seems to confirm that and my memory is not playing tricks.
https://www.bmwusfactory.com/manufacturing/production-overview/
With automotive water based paint systems in manufacturing plants all colours are clear over base. Primers and basecoat paints are waterbased. Apart from a few processes that used water slurry clearcoats (mainly Daimler) in the 1990s and some experimental powder coat clearcoat systems (including BMW) clearcoats are solvent based.
So your original paint would be what you are familiar with and your usual cleaning/polishing routine should work fine. If there have been repairs, or rhe car has been repainted, you may find that the surface responds differently to the original paint.
Hope that helps.