Would stronger equate to more reliable. Will last 3,4,500,000 before the engine conks out.
No car will last that long without reasonable and regular care and feeding. A petrol engine will be dead in the 200k range, where a diesel, taken care of, will run almost good as new at that mileage. You do need to adjust valves, deal with injectors, glow plugs, gaskets, seals and other normal wear items to get big miles. Transmission death around 200k is a given, and will be expensive enough to doom a car. Any car.
The w114/115 has a much lighter body, so gets more bang for the HP. The w123 is massive and taxes the lighter engines (240D, 230). The W123 got turbo bump to compensate for loss of giddy-up in the early 1980's. The 1976 300D (w115) is a "faster" car than the 77-81 w123. A manual transmission will increase power at the pedal.
Reliable in relation to a 30/40 year old car is subjective. If the previous occupants took very good care and serviced it properly, then it would be VERY reliable. An abused and misused car will be good for parts mostly. The maintenance cost for the car will normally be in the $1k per annum range. Some times more, some less. Depends on what breaks/ages out.
I like to get fresh filters and fluids for all new to me cars. All fuses get replaced with new (cheap insurance). That way I have a baseline to work with. Belts and hoses are next on the list if they look aged. Pull up all carpets and padding to find rust that is simple to deal with. Bad rust is a no go on purchase.
Known issues for a /8 are rust in the cabin corners and floor. The front fender has a mud shield that holds wet muck against the metal. This rots out the corners at the front. The leaking windscreen seals ooze water into both the rear footwell as well as front wells. Trunk also sucks up water which leads to rusted floor. Poor seals at the headlights can rust the bucket behind them.
Climate control is very simple and almost impossible to mess up. No vacuum to screw with. The W123 has vac pots and electronic that are an issue when they fail. The big fail point for the /8 is the heater fan. The car is built around it. Shop time to fix is 15 hours. Luckily, the system is analog and forward movement will push air over the heat exchanger, so you get some heat at all times. The fan just moves air for the defog.
If you can live without AC and the "modern" features, the w115 is my choice. Yank the AC and the rest so you do not need to repair them.
