I don't see that its needed, so the floor slopes a bit 1 inch over how many feet, for older houses that would no be unusual.
As for the laminate flooring it is generally a floating floor, so no issues there as long as there are no moisture problems.
Just use a larger baseboard to cover the gap around the floor.
I've used the self leveling cement product with great results in the past. One was a remodle on a Victorian the framer screwed up and had the floor joist off by an inch across 5 feet.
cost about $25 per bag used 4 bags in a small bathroom, so you are looking at 10 to 20 bags of material $250 to $500 in material cost, and two to three people for about 4 hours labor. For that much material I would use a trash can to mix it in larg batches,