As Jocko says, it could just be a anti roll bar droplink, a cheap and easy fix - 5-10 minutes. Even if it's a wishbone bush, it's not a major job.
As suggested above, take it somewhere ,tell them you have a knock, and get a quote. You can always go back to the dealer with it, I'd be surprised if he didn't stump up, or fix it himself.
As for the rust, it depends how old the car is, and where the rust is. Ours is 10 years old and there are signs of rust, but nothing to worry about. The main areas that will give you problems in the near future are corroded brake pipes and significant rust on the actual bodyshell, particularly around body sills and suspension and subframe mounting areas.