6.9's are not particularly unreliable or complex. I've bought a few that sat a while and drove them daily with little effort. The M100 engine is like a giant M117 and the suspension is not too different to a diesel wagon's rear setup.

Of course, like any other MB, if you buy one that's a complete basket case then it will eat you alive.

Find one with a solid drive train (engine trans and rear end) and you could probably fix it up at reasonable cost.

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With best regards

Al