well in my opinion, (And I dont know to much about Z's, other than what I have been reading here)
As far as Nismo's go:
a used 2009 with low miles under 5k, is worth about 29-30k.
a new 2010, is worth about 35k.
The hardest part like you said is that I am looking for only one color so my options are limited, but I am not in any hurry so I guess time is on my side.