I have a 2017 GLC 250d now on 122 000km and have never needed to do anything other than regular service items. I was faced with the same decision you have to make now. I took the money and invested it as a buffer against possible repairs and it's even grown a bit. This is something you will need to decide based on your own risk appetite profile. Not sure which engine you have but my independent mechanic with a lot of experience with these cars convinced me not to take a warranty. I recently saw a Vito with basically the same engine as mine still doing duty whilst having more than 600 000km on the clock.