is this psu too big for this build?
Its quite the headroom for the build yes.
Unless you were planning to add a second GPU (where 650W would be a better pick), I would cut the PSU down to a 550W.
That being said, if you are getting a good deal for it, it wont negatively affect your PC if you have a bigger PSU.
750W could be interessting if you plan for sli in the future. for 1 gpu 550W is more then enough.
There is no such thing as too big of a PSU, the PSU will only draw what it needs.
isn't running you psu at %80 better?
In terms of efficiency, yes, but otherwise, no.
Depends on how much you pay for it compared to other PSUs. Unless you found a good sale/deal on it, you probably ought to get the ~500W one instead if it's cheaper.
For example, I bought my EVGA SuperNOVA 750B2 when it was on sale for $56.99. My current build (let alone the Pentium & 750ti I used to have in it) won't even come close to 750w, but I couldn't have bought a better PSU for that price.
Doesn't matter. There's no such a thing as a too large power supply.