You say "rust" re downpipe.
o The downpipe is actually a low grade stainless steel. It
is non-magnetic and doesn't rust easily. Mine on a 1993V6
is still "white", apart from the welds which tack the two
halves of what looks like a pressing. That is despite 9yrs
of numerous winter/salt/UK-rain cycles. It sort of goes a
dusty bleached white metal colour.
o Now the /welds/ on the downpipe WILL rust, crusty orange
delineage of the two halves and what looks like tack bits.
o The flex-pipe is surrounded by a braiding which goes to
a clamp at each end, those clamps can rust through as mine
has. No leaks whatsoever and they'd be obvious & audible.
Thus:
o If the downpipe is leaking, it will not pass emissions
o Weld rust or surface rust is immaterial until it does,
and that pipe is actually very thick & heavy as a warning.
My Borla flange plate is bare steel and has thick crusts
of rust flaking off it's ~10mm thickness. Will last decades.
Other Sources:
o Email Roebuck/Trussville Mazda for the parts price, their
price may be quite a bit less than you have been quoted. It
is just a pipe after all and not a whole cat-back/muffler.
Since that piping is low grade stainless any shop should be
able to reweld it. I can't recall what grade, T4xx IIRC. Not
the T304 or (medical) T316 you associate with stainless. I
think partly because T304 is more susceptible to cracking
in "header" applications which this is to some extent.
Another Source:
Ask on
http://www.Probetalk.com or here for anyone who has changed
from stock to headers, they may have downpipes available.