I'm considering buying a 2006 BMW M4 Roadster with 64,000 miles on it. Are these cars known to be problematic or expensive to repair?

Not so problematic, but yes expensive to maintain and repair.

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!