[pgaiduk]

I am working with an old concrete patio in northeast Ohio. This patio is under a shelter roof so it might experience windblown snow but no direct weather.
I would like to install a grouted flagstone floor on this surface but am very unsure of how to proceed.
The 144 square foot floor has a footer around it and has been very stable.
One concern is that the floor is made up of the original smooth concrete pour and at some point was enlarged with additional 3 foot wide pour all the way around. At times I will observe moisture on the surface of the smooth original pour but not elsewhere. I believe this to be condensation due to temperature change and not ground water but some thermal condition under the original pour causes it to want to sweat on humid days or during temperature changes. (8 years ago we had it raised and leveled by pumping "grout" under it. Maybe this grout acts as insulation?)
I am not a mason but have laid many tile floors and built a few stone walls.
I would like to keep this floor as thin as possible due to the door immediately off the patio.

How should I prepare this surface for mortar?
Pressure wash? Acid etch? Surface treatment?
Or should I not bond to the surface and lay mortar on a moisture barrier and make a floating floor?
What kind of mortar? Notched trowel?
I would appreciate any and all feedback.
Thanks in advance.