You can get the cheap Chinese version of the durametric on ebay for about $20. this for example:
Newest Piwis Cable V3 0 15 0 for Porsche Can Access All of The Systems | eBay
This is the early version of the software and cable but works quite well to diagnose the engine and it's electronics. Do not try to use the later version of the software, it will render that cable useless.
This can give you all of the info you need on camshaft deviations, etc... This can help with diagnostics.
Is the car now a runner?? he's selling because he doesn't want to fix this issue?? That part can be changed in car and takes decent skills, but not super skills. Volvo and Porsche, in my mind will be quite similar for this type of work.
Good luck and the Box is a special car and worth some investment of time and $$.
I searched the solenoid part number and came up with the best price at about $165.00
http://www.************************************************************************.com/oe-porsche/99660590100