Solid Hardwood, Engineered and Laminate Flooring - Leveling Issue and replacement

Have an older house and over the years settleing of the floor has occurred and is uneven and slopes.

I would like to replace the flooring that is down, maybe even use the current flooring but before I do that I know the subfloor will have to be leveled. My questions

1) What options are available for leveling a subfloor that slopes across the room?

2) If leveler be used on top of exsisting sub floor :

a) What are its limitations as respect to amount of buildup needed to level floor?
b) What type of leveler is best?
c) Is there a certain type of certification a flooring contractor will have that I can look for in selecting one who has experience in this area?

3) If a new subfloor is needed, what are the best materials to use for the least amount of squeaking?


Thanks in advance and any links or info that you can offer would be great.