Hi and welcome OP.
There is loads of helpful information on here, including this buying guide:- https://z4-forum.com/forum/viewtopic.php?f=2&t=21281.
If you are going for a 6 cylinder one you might as well stick to the 3.0 litre as the running costs are the same!
Good luck finding the right one, and once you do, don.t forget the photos!