RepairPal Certified shops can diagnose and repair your 2006 BMW Z4. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
Vanos system can be a problem - listen for a rattling noise around 2800-3000 rpm. There is a lot of info on this available on line. At least 2 suppliers can provide repair kits for do it yourselfers. Otherwise they're great cars!