A big notched trowel is fine, remember it's purpose is to scrape
"key" the thinset mix into the existing cement. Your flat trowel
is what you will finish it with. It only needs to be as thick as
you need to level the floor...more where you have a dip, and less
where you have a hump....
I would use a workable mix, as you say "pancake batter", but
not weak or runny. Maybe more like soft peanut butter. A dry
mix would be hard for you to work with.
A good "pre-mixed" dry mix would be wall mud, which is what
is used to make "mud job" shower jobs. It has sand, cement,
carotex (sp?) which makes it sticky, and light particles like
styrofoam tiny balls or formiculite (sp?) to be a filler and make
it light. It's easily spreadable and easy to work with, and shouldn't
cost more than multi purpose thinset. It sticks like crazy, and is
strong in the long run (try to tear out a mud wall sometime).

Remember, all my ideas are jury-riggs meant to patch your
floor, not professional advice. Outside of professional tiling,
I often jury rig my own projects, so I know where you are coming
from, but you must assume responsibility for not tearing out the
floor, dealing with your water problem, or pouring a new slab.
The principles behind my ideas are to 1) have a mix that will
allow water to pass through and evaporate, thereby not trapping
the moisture underneath, and 2) make a mix that is sticky enough
to bond as long as possible, yet be strong also.
I try to do high quality jury riggs :-)

thetiler