My husband used to lay tile. You can find a product that you put on before the self leveler, kind of like a primer. Look into aqua d or Red guard. The added benefit is if you run them a couple inches up the side of the walls they provide an excellent water barrier for any future leaks you have.

Be prepared depending on the size of your space for multiple expensive 5 gallon buckets (unless you choose to strip, but I would still use it). Similarly, with the self-leveler you're going to find that it requires a lot more than you would imagine to get yourself a nice level floor. But it will be worth it.