Structural problems in a residential building

I am about to buy a flat and I am concerned about some defects. The building was built c.1900 and is a mid terrace 3 storey building with a lower-ground floor (so fairly deep foundations?) and butterfly roof. The ceiling and joist is visibly bowing with shadowing marks, assumed to be from a lack of insulation, also, the floor is noticeably sloped. There are a few minor cracks in the plaster near the window. There are no external cracks and the windows and doors are straight. Should I be concerned about subsidence? The flat below (Ground Floor & Basement) looks ok, so it could be a roof issue? There are no structural walls in the flat but it is fairly small and I assume the roof doesn't need supporting by internal structural walls.