It is the quality of the psu, not the efficiency rating that is most important.
A cheap PSU will be made of substandard components. It will not have safety and overload protections.
If it fails under load, it can destroy anything it is connected to.
It will deliver advertised power only at room temperatures, not at higher temperatures found when installed in a case.
The wattage will be delivered on the 3 and 5v rails, not on the 12v rails where modern parts
like the CPU and Graphics cards need it. What power is delivered may fluctuate and cause instability
issues that are hard to diagnose.
The fan will need to spin up higher to cool it, making it noisy.
A cheap PSU can become very expensive. Do not buy one.
The cpu and gpu are appropriate. A GTX960 needs about a 430w psu.
Again, it is not the wattage that is important, but the 12v amps.
Since the pc you are considering is already working, I see no big problem.
If building one from scratch, I would have used a 2 x 4gb kit of ram to get faster dual channel operation.
I think 120gb is too small today for windows. I would have used 240 or even 500gb ssd and omitted the hard drive.
You could consider building based on used parts.
If you have access to ebay, look for your parts in used condition. Filter on completed auctions and in green, you will see what they actually sold for.
As to the value of the proposed purchase, I think you are not getting a sufficient discount for used. Used parts will sell for 50-70% of new.
I think you can drive a better bargain if you are willing to walk away.