I'm looking at a property the was built in the lat 1800s. It's a 3 level brick building with two other building of similar stature on each side. I too a quick look at the second floor, and it pitches quite a bit throughout. (probably 5inches for 10 feet in various directions). I'm wondering what options I would have for leveling the floor in this type of structure. I think most of the movement is simply from 100 years of settling.
Huge RED FLAG ! 5 or more inches of floor displacement is a condition for serious concern. There are a number of reasons that could cause this condition. If you are not experienced in structural inspections, I'd highly recommend getting an inspector or structural engineer in there before you buy a serious problem.
The fact that it is also sharing common walls with other units complicates some repair scenarios. Get a pro in there and also get quotes based on the engineer's recommendations.
6 inches seems a bit excessive. I would double check if the foundation is in a good shape. I have a brick building of same age and all the floors are pretty straight. If the load bearing walls moved this much I would expect it to be costly and difficult to straighten the floors. I would also worry about structural integrity of the building.
I also noticed that you mention that it pitches in various directions. Is it pitching away from the brick walls? It is possible that joists do not span the full distance between the load bearing sidewalls and meet at a perpendicular beam. Maybe someone removed a load bearing interior wall at some point which caused the beam to sag. I think in this case it would be easier to level the floors