should've slapped a home insurance on it when you first got it. they would, more than likely, ponied up for that a/c.
re: ceiling. can they put a beam and make it not look out of place now? i think, in my non-eng head, that could be the easiest fix. they can also place some kind of support in the attic. i don't believe either of this should break your bank.
re: roof. 20 years on what shingle/type of roof? if your insurance didn't balk at it, then you're good to go. worst-case, you're out of your deductible and will have a new roof. the contractor part is the trickiest, so be prepared to shop around when time comes.
re: plumbing. how old is the house? i assume it's on crawl space with more-or-less easy access to all the piping. i find it hard to believe that's a real quote. i think someone is trying to milk you because of your expensive home.
basically, all this stuff is what an average homeowner should be prepared for when getting a house (even new(er) ones).