The problems with the Yanmar engine and Mercruiser leg combination are well documented, with both major components suffering from failures.
If you think Yamaha parts are expensive, go on YBW Forums and find out what Volvo owners think about parts cost and quality of service.
There aren't many of Yamaha's new 315HP ME432 installed yet, but users of them are very enthusiastic. Unlike the 315HP Yanmar unit, there's very little turbo lag, allegedly.
http://www.yamaha-motor.co.uk/produc.../ME432_TRP.jsp