I hate to say it but I have fixed many a cam - rocker arm problem with just rocker arms (a definite MB no-no, they say replace both everytime).
There is good reason for replacing both but if you know what your looking at it can be decided whether the cam has wore through its surface hardening. The rocker arms should be removed and viewed by someone with experience. The noise starts as the surface of the rocker wears through its surface hardening. This appears as a line across the beginning of the contact patch. At first outset the line is only visable. As it starts making noise it digs a groove all the way across. If fixed at this point the cam may have some surface roughness but no change of shape. This situation can have long term results (at least 50k miles)with just rockers. If the cam has changed shape its too late for it. Replacing the rockers will give temporary relief but maybe for less than 10k miles - not an efficient repair/patch whatever.
I don't know if I would condemn used cams so quickly. Until a rocker has failed under it the cams last forever.
I also would warn against forsaking used to buy a "new" cam from an aftermarket supplier. There are plenty of regrinds being sold as new and there has been a rash of talk on the tech only sites about misground new cams from aftermarket sources (good ones too).
A misground cam is a technicians nightmare. It is almost impossible to diagnose. It has to be done by intuition and only someone that would know the history and whats been going on would have a chance at that diagnosis.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician