Yes you should be concerned. You should assume there is internal rot, until proven otherwise. If it lasted long enough to create stains, then there will be rot. Unless they opened up the ceiling and fixed the rot.
And the next question is, how did they fix the leaks? If they caulked over the top of the suspected leaks, that is not a fix. Well, it can be of course, but the caulk on the top is only the first line of defense. And usually that approach fails. There is putty tape inside the mating surfaces which has also failed. A real leak repair involves disassembly of the leaking joint, new putty tape, then new roof caulk over the top.
I'm speaking from experience. If you buy it with the thought of fixing it yourself, you'd better get it very cheap and be ready to do a lot more work than it looks like.