Assume lead until tested. Paint dust from older buildings can carry it, and that changes how the surface is prepared and how the room is contained while the work runs, which is a reason to plan the job properly, not a reason to leave a wall as it is and hope the next owner deals with it. It is ordinary work once it is known about.
The surface work is what the price is really buying. Cracks and nail pops filled and sanded flat, glossy old trim scuffed so the new coat keys into it, and any patch large enough to notice primed before color. An unprimed patch flashes through in raking light and cannot be corrected afterwards without redoing the wall.
Humidity is the constant. Rooms closed through a Gulf South summer hold far more moisture than owners expect, and it shows on shaded walls and inside closets as a coating that will not hold. We look for the cause first. Paint over active moisture lasts a season.
Deep original trim and tall doors mean this is hand work, not spray work. Cut-ins by brush, hardware off instead of being cut around, everything that stays in the room covered, and the room put back together each evening so you keep somewhere to live while the job runs.