Head gasket runs about $60. The installation rate will depend on the mechanic's hourly and whether he has a boat payment coming up!
The timing chain tensioner and guide rails are service items for the 190E. There was even a factory upgrade notice issued to the MB service techs. Luckily, we don't have the belts like some GM and Honda cars, so the chain will usually not need replacement, but the tensioner and guides, that keep the chain from slipping and thereby ruining a valve or two, do need to be kept up. If you do a Search on timing chain, you should find everything you need.