If Merc assistance is anything like BMW assist then yes, they will carry a range of common parts to get you going, and they will be fully trained technicians.

A friend had an EGR valve failure on his 520d a few months ago, on a Saturday evening in Chester. Called BMW assist on the car, they diagnosed the fault remotely on the diagnostics, the engineer was with him 40 minutes later.

One hour after that, he was on his way. EGR valve changed and all diagnostics reset there on the roadside, by a highly professional engineer.