So in total new all of this would be worth about $1200 USD or £827.
Now what I reccomend is changing the CPU cooler and the GPU, the 770 is so expensive because they are no longer being made but the 970 which is about $330 is more powerful and many consider the R9 more powerful at about the same price. So I'll list again what I'd reccomend, and this would be new so a higher life expectancy and trustworthy refunds or help with issues.
Saying that everything else is the same - R9 390 $330 - h100i $100 This saves $290 or £200 which prices this all new and better performance at £627 all new and better performance.
You would have to get a case, power supply, and motherboard though but I'm not sure if those were included in the preowned items? Either way, I'd build your own if I were you as there's always liability to buying used items. With the added costs buying new may be a bit more expensive but you'd most likely get a better quality and higher performing machine. If you plan to game you could always go with a Skylake i5 which is $260 or £180. This saves about £80 and would still be amazing for gaming. In total this would save about £110 with all of my suggestions and the case, power supply, and MoBo shouldn't cost more than £160. So altogether if I got everything right, you may spend about £50 more but you'd get newer and better quality with the added enjoyment of building the PC! This is assuming that the preowned PC dos include the case, MoBo, and power supply.
Hoping I helped some! I know it was a very long response but thanks if you read the whole thing! Good luck in any path you go down!