Narco seem to have a reputation for being less robust than their well known US competitor, but the Mk12 is one of the most popular nav/coms in GA land. I've never personally had any problems with their stuff other than the little all-in-one VOR receiver and indicator.
The upgrade to 760 channel costs about £1200 but I think it also gives you FM immunity. The only time Ive needed to tune a >136.00MHz station was for ATIS, so I just used my handheld.
I think 760 may be compulsory in some countries (Germany?)