louie12 said:
Must admit I lean more to the Silver as it's being sold by a reputable indie and so potentially a safer purchase. So you wouldn't be put off by the mileage? As a rule is there anything pricey just round the corner?
The mileage wouldn't put me off IF the price reflects it (my air-cooled 911 had 191k miles!). Clutches seem to last and you don't have a cambelt to worry about which are two typically large bills you encounter with some cars at 70-80k/10 years. At this age details like tyres, brakes and inspection service can make a difference of hundreds or thousands. UK buyers are often mileage sensitive, so come resale time some will immediately rule out buying what they see as a high mileage car (you are having the same doubts).
Having worked in the motor trade I would buy from a dealer
but only if the price was close to what I could buy privately for or it was an exceptional car. Both the cars you have linked to appear nicely specced but neither stands out as exceptional value to me from what little you can tell in an advert. Cars sold by dealers are after all private cars that are being sold to make a profit/cover overheads, hence my preference for a private purchase when you can actually talk to the owner.
I don't know any of these cars but a quick search has shown a selection at a cheaper price/lower miles and I'd look to haggle plenty more off too. The Sepang one stands out to me personally at £1500 less than the black one you found and the same price as the 82k mile silver grey. I'd try to have a friendly haggle for another £500 off too.
http://www.autotrader.co.uk/classified/advert/201607195980736?onesearchad=Used&onesearchad=Nearly%20New&onesearchad=New&radius=1500&postcode=sn139fy&advertising-location=at_cars&sort=price-asc&make=BMW&model=Z4M&page=2
Blue, £14,980, 54k miles, dealer
http://www.autotrader.co.uk/classified/advert/201610178844121?model=Z4M&postcode=sn139fy&sort=price-asc&onesearchad=Used&onesearchad=Nearly%20New&onesearchad=New&advertising-location=at_cars&make=BMW&radius=1501&page=1
Rare Sepang, £14,000, 63k, private, Inspection 2 at 55kmiles and decent tyres, description suggests Z4 enthusiast.
http://www.autotrader.co.uk/classified/advert/201610299241660?model=Z4M&postcode=sn139fy&sort=price-asc&onesearchad=Used&onesearchad=Nearly%20New&onesearchad=New&advertising-location=at_cars&make=BMW&radius=1501&page=1
Also blue with cream, £13,995, 60k, dealer
http://www.autotrader.co.uk/classified/advert/201610028353820?model=Z4M&postcode=sn139fy&sort=price-asc&onesearchad=Used&onesearchad=Nearly%20New&onesearchad=New&advertising-location=at_cars&make=BMW&radius=1501&page=1
Blue with cream and CSLs, £13,795, 61k miles, private
http://www.ebay.co.uk/itm/2008-57-BMW-Z4-3-2-M-Roadster-Red-/272335786181?hash=item3f687a50c5:g:ytoAAOSwdzVXtJPS
Red, £13,250, 76k miles, 2008, dealer