Have you tried replacing the valve stem seals? Quick, cheap and easy and just might solve the problem.

Pulling the heads is within the DIY realm. The only special tools you absolutely need are offset socket bit for the cylinder head bolts and some way to pull the chain guide rail pins.

It's probably worth replacing the water pump and lower end of the timing chain while you have the heads off.

Sixto
91 300SE
87 300SDL
81 300SD
78 450SEL (sold)