While the aftermarket stainless steel cat-backs are going to be less prone to road salt corrosion, be aware that they're all going to be louder than stock.

Walker is a well-known maker of OEM-style mufflers and I would expect most muffler shops to carry their line.
http://www.walkerexhaust.com/catalog...letypescrn.asp

The Walker system may be all mild steel.
If so, it will still be prone to rusting out.
But it should only run you about $350.

$700 is highway robbery; keep looking around for a shop that actually wants your business.