I am not an expert in this area. But I have used leveling compound in
If the area is small (let say the size of a small bathroom), you can
use leveling compound to level the floor.
If the area is anything but small, you cannot rely on the leveling
compound to "self-level" because it will not self-level in anything
but a small area. You will have to manually spread out the leveling
If you are going to build a frame to surround the area that you want
to level (as what you have said), you may not need leveling compound.
You can use concrete that is MUCH cheaper than leveling compound. You
can use a long piece of straight 2x4 and rest it on the frame, and
kind of moving it back and fro to level it. This should be good enough
for ceramic tiles and such. If this is still not smooth enough for
your flooring material (such as vinyl flooring), you can go back and
pour leveling compound over it. You will use much less leveling
compound in this way, and leveling compound is very expensive.
Even if you use leveling compound, you may still need a long flat
trowel to smooth it. Better off get it ready and use it as needed (and
refund it if not needed), than wanting to use it and not have it
(meanwhile the expensive leveling compound is setting up right in
front of you).
Hope this helps.