To the O/P l agree, not sure any non-M Z3 is worth that kind of money unless it's exceptional, say under 10,000 miles.
That one has only four stamps in the service book so there must be a fair time gap between them, my near concours 56,000 mile 3.0 M Sport has a FSH but a five year gap where it covered few miles and it put some off paying half that price when l tried to sell it so what does that do for the £15,000 one?
lnsurance on a Z3 needs to be "Agreed Value" if possible.
As prices are going up for good examples but the car is still in the price guides at banger money.
Older BMW's are starting to get into the silly money category, look at (for example) the 635csi, 2002, and E28 3 series.
Eventually the Z3 will go up particularly six cylinder versions, but only for the best condition examples.
So if yours is a minter get it insured Agreed Value.
Some companies now offer "Modern Classic" policies which cover the Z3.