The Vanagon exhaust system will go a long way.  Moisture condensation
internally is the main cause of corrosion and failure of mufflers, and
the short length of the pipes on the Vanagon keep this to a minimum,
moisture evaporates quickly.  The muffler will eventually rust out, but
the pipes crack before they rust.  Welding can extend their life, but
they will keep cracking due to brittleness from heat.

A good quality CAT will last indefinitely if it is not abused by a
poorly running engine.  Mine has over 200,000 miles and is still fine,
although the flange did crack off but I welded it back together.  When
you replace your muffler remove it with the cat and look into the cat
with the muffler removed.  You should see  light through the mesh, if,
indeed you have anything at all in there.  Some don't, and came that way
from the factory because they passed the current standards without it.

The exhaust system is one of the longer lived parts of the system,
albeit somewhat expensive to replace.
--
Stuart MacMillan

'84 Westy
'65 MGB
'73 MGB GT