I do not think the typical failure mode of the 3.5 leaves you stranded. More likely the base oil consumption starts to gradually increase over time. At some point it is no longer manageable. You slowly lose effective compression in that cylinder as well.
For what it is worth having a really good or reconditioned lower block ready is not a bad ideal. You would want to use your newer series less trouble prone 3.5 head on the lower end of the 3 litre anyways. I would not do anything headwise with the original 3 litre head. Although you could sell it to offset some of your cost if it is pretty good and not cracked.
If I owned a 3.5 it would be prudent to have a 3 litre block in my posession as well. At some point soon enough they may become very scarce. I also would not even contemplate the change until the 3.5 was becoming symptomatic. A quart every three thousand miles does not really justify it. Also do not worry about a couple of hundred thousand miles if you think the odometer is correct. With the amount of tampering and outright failue of the odometers on mercedes that is probably low milage compared to most examples.