a baty, Concrete does not like to adhere to a pre existing piece of concrete unless you drill the holes and install the rebar. Appling concrete on top of another slab will work IF the new applied concrete is think enough over the old slab. Say one inch or so will only sluff off eventually and scale off also. Then any debris, grime and stuff on the existing will only make the project more dificult.
Now, others may disgree, but if it was mine, I would go down and rent a remote jack hammer from a home center and tear up that old slab. Then begin again fresh. You know, as we hear so often on this site, people frustrated with cracks and defects in their garage flooring.
Consider a good drain, a slight fall, even high sides and inserted bolts for the sole plate two by four and do this thing right from the start.
I am in Dallas, not too far from you. If your near Silsby or Village Creek, your in some beautiful country.