this is only natural. Earth gets dense with water and condenses with lack of water and then things on it will rise and fall, just like sections of sidewalk. That is why we have expansion joints.
Anyway. You must fix the drainage problem and have or manage water to run away from all sides of the home. The house has to sit on a mountain of its own. make the water go away with sandy loam soil and packing.
Now, to live with slight foundation shifts, you must improvise to an extent. Keep this in the back of your mind, homes rise and fall with the seasons, so to repair a slight crack in the wall above a door might be futile as it will close in the fall and winter and open in the summer. Live with these.
Latex caulk will work nicely, make sure its paintable caulk.
Now for doors dragging, I personally hate to shave doors at all, I would rather perform operations on the hinges to move doors up and down. Putting shims in the bottom hinge will raise a door and in the top hinge will lower a door. There is an intriquite combination with the hinges to correct any problem there.
I only shave doors when carpet issues arise, then immediately seal that new cut.
If you are still concerned, hire a structural engineer to take a look see before buying. Its a good idea. But, then any home you will find will have its settlement issues some worse than others. I am on a job currently with a home that was just completely jacked up and that home's attached garage was so canted that the garage doors would not go up and down.