I think the spec boxster guys run 235/40/17 up front on the stock 7x17 wheels, and 255/40/17 on the stock 8.5x17 wheels. I like to run 225/45/17 on the front OEM wheels and 255/40/17 on the stock back sizes.
For tires, I am like a broken record, I like the hankook R-S3, 200 TW tire, good on track and lasts a long time. I have a trailer and sometimes drive my street tires to/from the track but I have got lazy and now bring them as spares and leave them on the trailer and drive on the R-S3 all the time
If this is a dedicated track only Boxster, and is only driven to/from the track, there are lots of good 200 TW tires, Bridgestone RE-71R, BFGoodrich Rival-S, Advan, etc. Then there are some street legal 100TW tires like Toyo R888 and Nitto NT-01.
What all can you get in Norway?