It is possible that you have a worn lobe on the camshaft causing the noise. You need to check this before assuming that it's lifters making the noise. To replace the lifters you need to remve the cam bearing brackets and then push the lifters out of the rocker arms. The snap ring does not come out. You should also check for wear on the cotact surface of the rocker arms. Usually the shims between the rocker arm and the valve are reused with now problem.
Mercedes Benz Master Tech