Good folks like Mike Murrell and Hitman X love their 300SELs so the M103 in W126 has a fan base. Among an ‘87 SDL, ‘91 300SE, ‘93 300SD and ‘95 S420, the 300SE was my least favorite. Fuel economy vs S420 was negligible for me (above 70 the S420 was more economical), high revs on the freeway gets tiring though it’s not loud or buzzy, can’t keep up with a V6 Camry, it’s really frustrating to sort when an M103 decides it doesn’t want to idle glass smooth. And if you’re in California, an M103 300SE will likely require test only station smog checks which where I am means limited discount and coupon options. Basically state emissions statistics brand it a gross polluter.

For 88-91 300SEs, everything about the W126 applies and everything about the M103 applies. I don’t think the M103 necessarily needs a new chain on interval like contemporary V8s. I think you can go based on elongation. Change it every 150K miles if you must but there aren’t a lot of M103 broken chain threads. Finding someone to service an R12 system is difficult. Shops would rather convert to R134a which generally diminishes cooling performance but shouldn’t be a deal killer in NorCal. I expect PBU service will be due, a side from the refrigerant system and vent actuators. The PBU just gets old and cranky. Quality rebuilt units are available and easy to install. Vent actuators less so but you can do most without pulling the dash.

If you’re looking at 92-93 300SEs, that’s a different car and engine altogether.

Sixto
98 E320s sedan and wagon