I think I'm looking at original floorboards, rotting underlayment on top of those, a delightful assortment of other boards on top of that, then Pergo flooring.
I'm unsure of the best way to proceed. My gut says to pull up the top layer of boards to remove that underlayment, but then I worry that that turns into ripping up the whole floor.
(Left Side cabinet)
(Right side wall)
Run your hand under the boards on the right and push into the surface to check for any rot. It doesn't look like rot but good to check since you are in here.
Then for the leveling.. I'd get some furing strips and some shims and a panel of plywood get get a level set. Your going to need to redo the trim in the front regardless and you want the washer to fit well so you'll need at least some furing strips and shims regardless.
Anything like a washer or a dishwasher that has pumps and shit that spins and a basin of water, you really want them level. But depending on the size of the washer and the hole, you might only be able to use shims.
How tight is the washer to the hole vertically?