I agree with the chaps here, I picked my Jan 2008 57 plate 3.0si Sport up a month ago. I got it from a local BMW main agent. It had 1 lady owner and had been supplied and serviced by them from new, 29,000 miles, Titanium silver paint with the brushed aluminium trim interior and 2 tone black and silver grey seats with matching silver grey door inserts (looks really tricky combined with the brushed ali dash and console etc). As stated already interior spec is very subjective, some people want them fully loaded others are not so bothered. Remember, with BMW just about everything is an option when you buy new! So with 2nd hand you are essentially buying someone elses preferences so although there are LOTS of cars out there you may need to decide what you are willing to compromise on. For instance, in my case it had to be the 3.0si sport with tiptronic and paddle shift and it had to be a silver coupe with low mileage, those were the things I would not compromise on. My car has no built in sat nav, no heated seats (they are elec memory however), no auto folding mirrors, no cd changer. But none of those things were deal breakers for me, it's got the things I really wanted (ok heated seats would be nice on cold mornings

) and on top of that they gave me 12 months extended warranty, fitted a brand new glove box, 2 new front fogs and 4 brand new 18" star spoke split rims. I paid £15,800 for it, which felt like a cracking deal.
So shop around, decide what you want and what you can live without, and flash the cash. You absolutely won't regret it, I promise you will love your Z4. And keep in touch here, I've found this site an amazing resource of information and like minded Z4 loons (sorry enthusiasts) and everyone has a sense of humour so whether you need serious info or just want to natter or let off steam this forum is the place!
In fact I'm quite addicted to it at the moment
Anyhow I'm off for a blast in my beautiful coupe now! Good luck
Al.