If there's concrete under the plywood then that kinda rules out nailing the engineered flooring down and leaves you with gluing it down.
Generally, engineered flooring will require a fairly flat surface, usually about 1/8" or less over a ten foot span. If there is much more variation than that (whether you nail it or glue it), you will get voids under the flooring and you can/will have problems.
I wouldn't use anything less than 1/2" material and would recommend using 3/4" thick flooring. This will be pricier but it will also (usually) give you better floor.
My diet is working good. I'm down to needing just one chair now.
"Just think how stupid the average person is, and then realize that half of them are even stupider!" --George Carlin