here is how i approach the subject.  the end of life will be when you can
no longer supply enough parts to keep your van running.  the general rule
of thumb is that the older the machine the smaller the quantity of parts
that will be available for it.  here is where it comes down to how
committed to the machine you are.  if you what the longest life and
lowest cost per mile you "need" to stock pile parts.  that means
purchasing replacement parts when they are cheap in advance of when you
need them.  additionally the cheapest source of parts is to buy a vanagon
and part it out.  the parts on hand are worth much more than the $300-500
that the van will cost you, of course i discount the labor to part the
van out and box everything in a organized fashion.

for example i have two spare transmissions and both vans that they came
out of cost me $500. i've already recouped at least the purchase price
from both vans so the transmissions are basically free at this point.

jonathan