I'll assume you're talking about the 88-91 model 300-SEL; not the one made 30 yrs. ago.
I bought a beautiful '91 300-SEL in 1997 with
35K on the clock. It now has 65K and to date, I've had zero problems. Fluid/filter changes are a breeze. The car features an M103 6 cyl engine and plugs are also easy to change. The 88-89 models had weak valve guides/seals(so I'm told). They leak oil out around the distributor area and some leak out the rear of the engine(head gasket). The guide/seal issue could cost you $1500.00 if it needs to be done.
In my opinion, the 300-SEL is a fine car.