Uneven floor and kitchen/bath reno
I have a 1920's house, about 24 x 16 (a 4-square/prairie hybrid, i think). The first floor has 3 rooms - kitchen and DR are the rear half of house with a wall between them. The floor in the kitchen and dining room are both uneven in that both curing somewhat towards the common wall. I got used to it but visitors notice it. The second story floors have no sag. In the basement there's nothing beneath the common wall.
I decided to redo the kitchen, mostly due to leaky pipe(s) in wall (not the kitchen/DR wall). The contractor said he will level out the kitchen floor (I plan to get wood). He is not an architect, however, so my first question is, do I need to get an architect or engineer to evaluate the floors before the kitchen guy starts? If I need to jack/sister(?) the sag (I don't know that means but read it here) is it ok to redo the kitchen and its floor now? Second question is, I also need to redo the bathroom that's right above the kitchen (no problems now but probably coming). In your view, what's better - do kitchen/bath reno together or spread them out, considering that it's an old house, has old iron pipes, etc.,?
Thanks for your thoughts,