I'd imagine things on the z4 that im doing would all be comparable costs?
Tyres would be more for the zed, they use the RE050A run flats and cost an eye watering amount considering how quickly they can wear on the back - M sport suspension also has a tendancy to eat up the inner edge of the front tyres too 225/45/18 and 255/35/18 cost ~£190/230 a pop

Weak points of the Z4 (non M), roof motor can be troublesome on earlier vehicles although mine is thankfully fine - big job to replace at main dealer prices (I use it often so it doesnt get chance to crud up). They also share the E46 weakpoint of rear coil springs snapping at random - bmw will usually contribute if you threaten them with enough bad publicity.
On the plus side they have long service intervals (16k miles variable) and the drive train and engines seem well matured. Its pretty much turn key motoring with brakes and consumables easy to replace yourself, only needs to see a dealer for a stamp in the book

Do you find the S2000 relaxing to drive on a long run?