Undercoating once the rust has already started won't really help much-- there are some actual rust treatments, but they are very expensive.
However, I'm not totally convinced from the original post that the car is necessarilly as bad as the other posters have been thinking. The exhaust rusting out is pretty much normal-- cars need new exhaust components sometimes! Car bodies (as long as they're not FIATs)usually fare quite well in rainy climates, and it could be that there's just a bit of normal surface rust on the bottom of the car. There's nothing in the post to suggest any kind of serious structural problem.
Still, best to get it checked by a professional!