I have built a foundation for my house and leveled it. I also releveled a house for a neighbor of mine. It wasn't that big of a deal, really. I did it in less than a day and by myself. I didn't build new piers, however, i just added shims on top of the existing piers. New piers or "drop girders" will add a lot to the time and effort and materials.
About your questions...
I guess it depends...
First of all...houses are REALLY FLEXIBLE THINGS. It's amazing. However, plaster and window glass are not flexible, so cracks may happen. I'd suggest opening windows during the levelling process. The window frames may "rack" and they'll need to be reworked a bit, but the glass is less likely to break.
As far as plaster goes, basically you just may have some cracks to repair when it's all done. Also, it is likely that some doors will no longer close until they are reworked, too. that happened to my neignbor's house. Leave the doors open while working to avoid getting them stuck closed or worse...shattered.
A sagging house may be a combination of: a sinking foundation (which leads to a whole wall or part of a wall dropping (and the floor with it)).
and: sagging floors where the floor is supported at the walls but sags in the middle, because it wasn't supported in the middle.
Gosh, it's hard to diagnose your house without seeing it. Sorry. It's really easy to see and describe in person.
It is possible that listing the floors will lift the roof. It is also possible that additions and the original house will all level up nicely without problems. If the fellow says that's your case, be happy. In my case, i had to push and pull from several directions simultaneously over the course of months and add drop girders, shims and attic supports to get my house straightened up. Due to my unwillingnesss to remove some internal walls, I had to stop before it was really straight and be happy with the progress I had made. It took me months and some very clever rigging (if I do say so myself). It would have cost me many thousands of dollars...and that's for a 900 sq ft square house that i had just placed on a perfectly level brand new foundation.
One cool thing that did happen was thet there was one door that was so stuck I hadn't yet gotten it to move and upon setting the house down on it's new foundation, i heard a creak and looked up to see the door swing closed and LATCH!