You could improve the system . You dint need the i7 haswell cpu . An ivy bridge i5 is just as good , haswell does not really offer any advantage to a gamer . A SSD in the build would be a big plus . At your budget I think a GTX760 Ti would be a good choice . Try looking at Scan.co.uk or PC Specialist . Both have custom build options. I will take a look later and post some more comprehensive suggestions.
OnePlus 5T review