How old is the house? In a hundred-year-old structure, some shifting is almost certain to have occurred and nothing should be expected to be perfectly square or level or straight.
If plaster isn't cracking (diagonal cracks being an especially good sign that things are moving unevenly), it's probably pretty stable in its current condition, even if that's somewhat tilted.
If there is an ongoing problem, an engineer can look at the stresses and advise on whether and where to place laly columns to stabilize it, or even to gradually lift it back to level. It might be worth spending the money for that expert analysis before you buy.
Sticking doors are pretty much universal in older buildings (see above), and are most often solved by trimming the door slightly or shimming the hinges to make things line up properly.
My dining room has one corner tilted enough that I am using wedges under the cabinet's feet to level it. My living room has a much subtler tilt that goes unnoticed unless you look too closely at ceiling heights in opposite corners. One of the bedrooms has a tilt that feels pretty bad walking around the empty room but goes almost unnoticed after furniture is brought in (and will be even less obvious when I level the bed). There is a step at the entrance to one hallway to level its floor. But given that the place dates back to 1890 or so, all of that is minor enough that I'm perfect content to just accept it as the history of the building.