Depending on condition, I'll refinish the existing - rust stains from nails / staples can be a problem, as can discoloration from water. Usually I'll do a quick bleach with oxalic acid, and see what it look like (if there is no finish. If there's finish I'll make a decison based on the way it looks without doing anything). Other options are shimming the floor from underneath to get it flat, then go over with floating floor, pulling the floor from above, leveling the framing, then put down whatever you need for what goes on top. I've done all of these - cutting out the floor next to a wall could require some support be put in the floor framing under the wall - you have to look under the walls parallel to the joists to see how they're supported, they may just be sitting on the boards with no joist directly under.
If you're lucky enough and he really wants a floating floor, you can use self leveling to get rid of any dips and be good to go. A little pricey, but much faster than other options.
Floor heights can be a problem, but you already know that.