Here is a link to an E85/E86 buyer's guide which may be helpful to you (and possibly the OP):- https://z4-forum.com/forum/viewtopic.php?f=2&t=21281
Good luck with your search.
And you really want a 6-cylinder one for the full Z4 experience! The bigger the better as the 2.2i, 2.5i and 3.0i are all the same engine, just in different sizes so running costs are pretty much the same for all 3!