Actually replacing exhaust isn't too hard, but removing a rusty exhaust can be pretty hard. If you have some PB Blaster and air tools, you should be able to handle it. If you only have hand tools, it might be tough.
I did mine myself, but nothing was rusty and I have access to a lift and air tools, so it was pretty easy.
If you have trouble, you might want to buy the pieces (y-pipe, resonator, muffler) and then pay a shop to cut off your old system, then put in some pipe for you from the cat back.
...i'd do that, but also get a mid-pipe, all the while keeping the stock muffler if you want everything to still be on the quiet side 
