Installing a muffler is fairly straight forward. Personally, I source all of my exhaust repairs out to the local garages, I can't be bothered with it. It'd probably be about an hours worth of labour to install one.
It's up to you to determine how much of the exhaust needs to be replaced, unless you can post a few photos, but even then you need to look for holes and cracks.
I do not have the vehicle yet, but there are no leaks before the muffler, it seems the muffler was hit on a bump or something. The reason i need a muffler on it (even if it is a cheap one) is that NJ does a tail pipe emissions test, and with the hole on the muffler it may not be able to pass due to the exhaust exiting out the side of the muffler.
if you havent bought the muffler yet screw napa, grab a summit racing turbo muffler, they are literlly $20, and they have a tiny bit more noise than stock, barely noticeable, they are a great cheap replacement.
If i do eventually replace the entire exhaust i will convert it to the 3.8L police version. But i will see what i can get ahold of in NJ for the summit exhasut (i am going to repair it all in a day due to the fact that its Inspection sticker was up back in Dec)