Why do you want to resurface? If there are issues relating to uneven, broken or sagging concrete then pouring something over the top of that won't fix the problem with whatever is underneath. If it is more related to mold/mildew/discoloration then a good powerwashing might resolve that.
I would assume anyone who gives you an estimate would first have to look at the condition of the current deck to make suggestions about what the actual issues are and how to best resolve them. Poor drainage or soil that has shifted over time needs to be addressed in order to solve this type of issue. Overlays won't solve a drainage or poor subsurface issues.
At a prior home, the builder had to resurface a patio (at their expense) since there was an ice storm right after they poured the original concrete. As I recall, they had to pour an additional 3-4" over the top of that so it wouldn't crack and create other issues. Worked out fine after that. I would avoid anyone who offers to do a thin 'resurfacing' layer since it would likely crack over time and creat a problem bigger then you are already trying to solve.