1. Your stock exhaust is press bent which limits flows in its bends while a mandrel bent exhaust keeps the same diameter piping throughout the whole exhaust system. It really depends on what you wanna do though because you're not looking at huge hp gains from switching to mandrel bent piping.

2. Here on TR, Craigslist, Ebay

3. I had an Apexi N1 on my Eclipse. Loved it, sounded great. I have an exhaust clip on youtube. http://www.youtube.com/watch?v=N-ryXsdmS8sl
Would recommend the muffler to anyone.

4. No need to go larger than 2.5"