you should be able to get a bumper from a salvage yard (in color too) for under $250 including shipping -check out www.car-part.com
and see if there are any available.
I changed the rear bumper on my brothers old 1992 SSE and it took me all of 1 hr with basic hand tools.
Fairly easy job.
As for the exhaust, you should be able to get your Flowmaster, high flow cat, and high flow resonator for under $500, if you buy the muffler from jegs ($100 including shipping -I'd suggest a Magnaflow F-body muffler and high flow cat from www.justmagnaflow.com
for under $150 including shipping), and then get a high flow resonator from a good exhaust shop (connect the Grand Prix club in your region and they should know of a good shop with good work and parts for cheap).
Good luck with everything. Also, for that price you might just pick up a good trunk and rear bumper from a 1996-1999 and get a new look. Should be able to get those for under $500 including the taillights.