-Callum wroteI was just looking at peoples opinions on a monitor and wasn't included in the pricing, that's purely for the build.
Well we'd really need to know your budget for the monitor. Or better yet, budget for everything including the monitor since you want to pair your system with a monitor that makes sense(and vice versa). I tried to stick as close to your £750 budget as possible with a reasonable monitor and system since you never gave us a budget.
-Callum wroteAs I say I think I'm going to try and source used parts
I wouldn't bother right now. The used market isn't great currently, and you're not really going to save much money by buying used. You'll just end up with older components that perform similarly for potentially a slight saving, but I'd rather just buy brand new parts with full warranties for the sake of 50 quid or so.
Seriously, if you bought a used R5 2600+B450 mobo, you'd save like £30, maybe £40. For a similar GPU to the GTX 1660 Super on the used market, you'd be looking at a 1070Ti and most people are asking silly money for their old, used cards- I see people asking £200 for 1070Ti's still. At best, you'd save like £25 by purchasing a used GPU.
There have been times where the used market makes more sense than buying new, but not right now IMHO.