Clutch sticking & whining. Undercarriage rust. :(

[sypher]

I bought my 03 Boxster S from an online auto wholesale company with 50k miles on it. The carfax report was clean but showed the car was from Minnesota. I had a PPI done. The report came back good except for a hand written note that said "Muffler has some corrosion" and "Surface corrosion on exhaust pipe." I didnt think much of it then and purchased the car.

Since then, every time i have had the car worked on, someone says, "wow, theres alot of rust under there. Car must be from up north". Yesterday my clutch was sticking on the way back up and there was a whiney scraping noise when the clutch was engaged so i took her to the shop. They told me the throw-out bearing was the issue and the clutch needed replacement (quoted $1400 for the job. OUCH!!!!) Then to make things worse, i get a call a few hours later from them saying that, because of rust, they couldnt get the muffler off without destroying it and they were trying to find a used muffler from a junk lot for me.

They said a new muffler from porsche is around $1200.. but i dont want to spend that. What can i expect for a used one? Is there better aftermarket alternatives? What other problems am I likely looking at from the rust? Is there a way for me to treat the rust problem to prevent future crap like this?