I can only comment on M54's. I have have rebuilt 3 of them: meaning new rings, crank bearings, etc.

As of now, I don't think you can can sell a properly rebuilt M54 for much of a premium over a used engine which is $500-$900, and to get $900+ I think you would have to guarantee them.

Now I think what you propose is a good idea and what you would end up with would be a near bullet -proof M54 with no oil consumption. It should be worth $900, but wouldn't sell that high as there are still too many used engines around.

Of course this all changes when/if M54's become scarce.