Check into a product called sika. It's good for cracks in concrete. If you want to add on top of the existing, you will need to score it so the new will adhere. Frame up and add at least 3". We had to do something similar when we converted a carport to a garage but it required way more than 3".
There’s several topping compounds at Home Depot and I would read their bags carefully, for your situation.
Yes, prep work will be critical.
You can't put concrete resurface on a deck, did you mean a concrete patio?