It's not pristine but pretty good and got 70k on it so less than 3k a year from new.
I believe it was also garaged by the previous owner for 15 years
Mines the other way round. It's garaged now but was outside for 15 years.
I got my z3 about 18 years ago with visions of me doing some top down driving - but around that time we got rid of the missus's fiesta and she just, kinda, took over the zed and I hardly got a look in for the next 15 years. No garage so parked outside. Driving through every weather and season doing 10 miles to her work and back through salted roads every day then parked outside on the drive.
Never hosed it down to get the road grime off. Didn't own a hosepipe. So all that salt just festered away under there till the rain washed (some of it) away.
So. Anyway. To cut a long story short. A couple of years ago I managed to get the car back off her and started taking care of it. Carpets out to dry it out. Footwells full of water but floors completely rust free. Sills off to inspect the damage and sure there's rust and a few holes needed welding but nothing major.
The places where salt gets behind the wheel arch liners and between the inner and outer sills. That salt is going to stay there because no amount of hosing will get in there and wash it out.
But seriously. For a 25 year old car that was used all those years all year round and never looked after there is surprisingly little rust. I've always had Mercs. But if a Merc was treated the same way as my zed for so many years there would be nothing but rust.
I've now stripped it down for a rebody and inspecting and fixing things that I find and I am gobsmacked by the z3 build quality and factory rust protection.