CPU can never be too powerful for graphic card -because it's power can be used outside graphic card performance. But if question would be "mostly for gaming", then I think 5600X would better choice for RTX 2070 (is cheaper and you won't notice the difference in games).
Yes, NH-U12S is good for 5800X (is one of the best mid-range coolers).
Yes 750W PSU is more than enough for 5800X+RTX2070 (actually even 650W would be fine) -or let me put it that way: better hi quality 650W PSU than "cheap" 750W PSU.