I dont think "Selective" replacement is a good idea because I don't see why 1 would be bad and not another. I also don't see why an intake one would be good but not an exhaust. These are wear parts and they all wear the same and are all the same age. Also, unless you're putting a fairly large amount of oil into the cylinder at all times (in other words, it doesnt get a chance to burn off/out), you won't have fouled plugs. Unlike many American engiens of this era, MB ones have quite self-cleaning combustion chambers.

I think your mechanic is showing he's a two-bit mechanic (in suggesting "Selective" replacement, he's trying to take a lazy man's way out. Same with suggesting just doing the exhausts). The seals come in a set of 4 (4 intakes and 4 exhausts). You need 2 per car. No need to throw the intake seals out. You're gonna have the cyl at TDC anyway, possibly pressurized with compressed air as well. The exhausts are harder to do than the intakes. You would be best off doing every one. Then, in two months or so, if it started doing it again, you wouldnt need to go in and replace the rest. That'd take MORE time to do!

Find a new mechanic and don't do a half-assed job - you'll regret it later on when you need to go in and do the rest. These seals are over 30 years old and rubber. I don't know how 1 30 y/o rubber seal could be good while another's shot!