Yep, as Sixto pointed out. I love mine due to the simple I6...like an E23 BMW, but realize it does nothing very well. It does not run off a stop light well. Fuel economy is easily similar as my 560, granted my 560 has true duals now off a 9:1 Euro motor so it does breathe MUCH easier if you have never seen that driver to passenger crossover pipe (1-5/8" OD, NO JOKE). It is somewhat noisy on the freeway but will run all day long at 60, 70, 80, 90. Whatever. I think it is out of gear around 125mph at 6400rpm area. I still give mine full power 3-4 upshifts around 90mph, car seems to like it.
I did manage to squeeze out 21mpg freeway going 65mph once with no headwind on I45 south and the AC off. I guess it averages 14-15mpg. City only in the summer barely gets out of the teens, maybe, as the AC is always on here in Texas and the fan clutch is roaring. Freeway summer tends to be high teens going about 70-75mph...limits are now 70mph in most of Texas so I have to try to keep up.
The SDL can damned near take it 0-60mph. M116/7s always need valve guides with age, otherwise the 420SEL is the best bargain around to me. Decent torque, decent top end HP, decent fuel economy. Less crap to break down compared to the 560. No SLS, no power rear seat.
My 300SEL also has been exempt from emissions for years, so speak with Duke on that. His knowledge how to pass that is NOT something to ignore.
BUT!
The M103 is a very simple engine. Yes, they leak oil. 97% can be fixed easily. A 25 year old front main is going to leak. The fan bearing bracket o-ring is often forgotten when replaced. If the lower cover leaks, spend the $40 and buy OEM sealant. Buy an OEM 'U' shaped upper timing cover gasket. Same for the valve cover gasket, the aftermarket are garbage. I finally became fed up with the TTY head bolts and went to ARP studs, my No 6 cylinder oil leak is gone.
Only use the OEM tensioner. Sachs fan clutch. Otherwise you will make yourself crazy. OEM motor mounts. Idler bushing replacement requires dropping exhaust.
The Superior Transmission valve body kit is a must for the M103/W126 to me. It allows you to tone down that crazy harsh 1-2 upshift. Then you will be able to make use of that gear if needed once you dial in line pressure and bowden slack. That will take some farting around to dial in exactly.
My catalyst was also partially plugged due to the oil consumption the thing had for years. I gutted it a few years back and power gains are noticeable. The vehicle is now much easier to roll out in second gear with the AC running and fan clutch locked. Head job at 275k area cured that and picked up nice power. Cold compression test was 160-170psi on all six cylinders and crosshatching was still nice at 310k area when I did ARP studs and OEM gasket.
It is easily the least expensive W126 to keep on the road that is not Diesel powered. Power application can be sorted once you figure out throttle input/trans shifting to keep it on the cam. The car cannot be lugged at 2000rpm like a V8, it simply has no power to do this.
Would I buy another? Probably if the deal was right.
Hope it helps!