750W is plenty for that system and enough to add another 670 on top.
If you dont plan to fill the motherboard RAM slots, that kit will be fine. If you might in the future, you will want low profile RAM as it wont conflict with the heatsink.
The paste that comes with the 212 EVO is re-branded MX-5. It should perform fine, and the differences between thermal paste's are minute at best, its mostly brand loyalty and perceived value that make people pick one or another.