The 300SEL uses the M103 six cylinder. It's the same engine as found in the 300E of that vintage. A very good, solid, smooth engine. There are two or three minor troublesome areas with the M103. Some of them do occasionally need a head gasket replacement. Oil leaks around the timing cover are somewhat common. And finally, the valve seals tended to wear early, resulting in much oil burning and blue smoke. This last one almost certainly has been addressed already. If it hasn't, probably best to avoid the car - burning oil for extended periods will cause other damage.

That's about it for the M103. The timing chain lasts a very long time - at least 200K miles in a reasonably cared for engine. Other than head gasket replacement, the engine should not require any mechanical work unitl it is past 200K miles. At that point a valve job may be required. Oh, the water pump is a pain in the knuckles to replace!

You can try searching for discussions on 300E engine problems for more info. The 300SEL (and 300SE) are somewhat uncommon, so you won't find too much past discussion about them here. Just be careful not to confuse issues with the M104 used in later 300E/E320 cars with the M103 used in the 300E from 1986-1992.

The 300SEL isn't exactly going to be a hotrod. It's a large, heavy car. The 3.0 liter six makes reasonably power, but only at high RPM. And the transmission probably starts in second gear. It ain't exactly going to pin you to the seat leaving stoplights.

- JimY