Well, first of all, K-Jetronic didn't come about until late 1974, so a W111 coupe` or cabriolet with the M-116 3.5 liter V-8 will not have K-Jetronic. These have D-Jetronic. D-Jetronic is all electric, and K-Jetronic is a mechanical continuous fuel injection.
Even so, I wouldn't worry too much. If the car you buy has been maintained well enough you won't have too much to worry about. I have been driving D-Jetronic cars as daily drivers for 25 years. The most expensive thing I've had to replace over the years is when my manifold pressure sensor went bad. It was $500 USD (5 years ago). Yes, the injectors are expensive, but they can be cleaned and brought back to life at minimal cost. Obviously, if the electromagnet inside of the injector goes open then you have to buy another.
The V-8 engines are more costly on upkeep all around. They hold more oil, 8 of everything instead of 6 (pistons, plugs, wires, valves, etc), and they are very thirsty engines in that they drink a lot of gas.
These early V-8 engines were bad about wearing their valve stem guides causing a lot of smoke from the exhaust. A lot of people try to cheat this by replacing just the valve stem seals. They quickly find out that doesn't take care of the smoking problem. The original guides were too soft, but the ones made today are harder and longer lived. Removing the cylinder heads to do this job is a $4,000-$5,000 job if you pay someone to do it, but it's worth it.
The timing chains in these V-8 engines stretch prematurely, and the chain rails in the cylinder heads are bad about becoming brittle and breaking......which will cause catastrophic damage when the timing chain comes off or slips a tooth or two. The rule of thumb regarding timing chains is to replace them at 100,000 mile intervals. On the V-8 an 80,000 interval is necessary.
The only other noteworthy issue with the 3.5 engine is all of the fuel lines/hoses going to the fuel rails and between the fuel injectors and fuel rails. These start leaking fuel, and it happens quickly and without warning. If the leak is bad enough it can contact the exhaust and start a fire. Many W111 coupe` and cabriolet, as well as W108/W109 cars, have met an early death due to fire. But don't let this turn you away from ownership. These hoses must be checked often and/or replaced at 8-10 year intervals. Too many people don't do this, and the resulting consequence is their own fault. Remember that rubber has a shelf life of only 18 years. Add in a lot of high temperatures from engine heat and the life span decreases. The same goes with something like a windshield seal. They have to be replaced every +/- 10 years. Nothing rubber is ever good for the life of the car.
Another issue that people never address is the intake manifold seals. Not necessarily the gaskets that go between the heads and the intake itself, but the round rubber seals between the upper and lower plenums. Again, they're rubber. They have to be replaced at +/- 10 year intervals. Otherwise you'll always have fuel mixture issues.
When all is right on a 3.5 engine they are an absolute blast to drive. They're very quick off the line (thanks to the 4 speed automatics that start off in 1st gear), and if paired with a standard shift the experience is even better. The fuel computers are solid state and rarely go bad, even 50+ years later. The most important things to remember are to keep the fuel system in check, and keep the engine cool. A 50 year old radiator may not be leaking, but the insides of them will usually be clogged.
For your knowledge, and to simplify things, from 1969 through the end of 1974 all 3.5 and 4.5 V-8 engines used D-Jetronic, but remember that the M-110 and M-114 were also offered with D-Jetronic, too. Volkswagen and Porsche also used D-Jetronic for a short time, as well.
In short, buy the best that you can find and afford. You won't be sorry, and don't give much thought to the electronics being 50+ years old. These electronics will outlast us.
