The rear seal can be done w/out removing the engine. It is definitely not a 3K job. Find an honest mechanic.
If the other oil leak blows smoke on the right hand side (facing forward) cylinder bank, then it sounds like you have a blown timing chain housing gasket. A reasonably easy fix but at the same time you should look into whether the car has had the single-row timing chain changed out for a double-row chain.
107 026 12 000677 - The original 'modern' Benz super coupe
107 024 12 011890