All of the engines will last just about the same amount of time/miles. But the gasser engines need extra maintenance when compared to the diesels. They are also a bit more complex (electronic fuel injection system and electric fuel pumps....idle controls....valves.....etc etc...) so in that regard there's more to go wrong/wear out. If well maintained they're probably equally reliable. The biggest downside to the gassers is the fuel consumption. Anything bigger than a 300SE Inline 6 or the 380 V8 will be a thirsty beast. The two I mentioned are relatively close to the diesels in overall consumption, when you factor in that diesel is 30-40 cents more per gallon than premium (which most MB gassers require) its about the same to run those. Its all personal preference. If I was to buy another W126 to add to my fleet I'd probably consider a 300SEL or 300SE....or even a bigger one (I'd just drive it less  )
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-diesel is not just a fuel, its a way of life- 
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
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'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
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