Padlin... Shifting seams would mean movement or shifting at the seems between aluminum panels... Leaking plumbing could be caused by freezing or sitting there all dried out...much depends on the climate and conditions under which the unit has been sitting... In general, just sitting there for years is a bad thing...and if it's vacant and not used, that's even worse, as things that just sit tend to deteriorate faster than when they are in use...it's that old "use it or lose it" thing.
Here's a link to a subform that will gve you some general things to consider when you are looking for a used unit: http://www.airforums.com/forums/f353/
And there are other subforms that deal with specific systems and issues...clink on "forum listings" in the left column of your screen...that will take you to a list of subforums... Poke around in there and you will learn a lot about what to look for and what to do about certain things... and of course, you can ask questions, too! That's allowed!
Good luck, and we hope to see you down the orad in your own Airsteam soon!