Can it help if I'll change only back tires from all-weather to snow? I have c230 kompressor...

 

yes it will help....but 4 tires are recommended.

 

Thank you! Going to buy 2 winter tires

The local diller recommends me to put a sand bag in the trunk...

 

Tires mainly do three things. Go, stop and turn. Winter tires on the rear only will help you go but rear tires only partially help you stop and have no input to turning. I strongly suggest you get four winter tires. Otherwise, your stopping and corning ability will be severely compromised.