So I finally decided to buy a Boxster this weekend. It's a '99 but the previous owner took good care of it (with all receipts to prove it). He only took it out of the garage when it was bright and sunny outside.
The next day, the plastic window cracked as I lowered the top
It was 7C outside (about 44F), which is above the 0C listed in the manual, but as I later discovered (from reading this forum), the manual is quite wrong.
I'm going to order a glass top from cabrioworld.