Rathean said:On the price appreciation you just have to look at mint Z3s or Z4Ms to see what rarity and condition do for value. It'll take time but good Z4s will eventually command a premium. Having said that I've been using mine as a daily since I got it and that's not about to change... I was toying with getting a 4x4 for the winter but I'd miss sitting in the z4 cockpit
Applaud your abundance of optimism
as the current values of Z4Ms are visibly faltering i can't even imagine how long its going to be before E89s become so desirable the values head north 
On the current 89 values , did you see the one "Phoenixboy" bought yesterday from a dealer ?
Full Bmw history 2011 Msport 23i 90k for £8995
https://www.ebay.co.uk/itm/BMW-Z4-sDrive23i-M-Sport-2011-11-reg/142959715182?hash=item214910236e:g:npEAAOSwYsBbs4jb