If you don't want to build it....and don't want to pay someone to build it for you......You're left with pre-builts.
You won't get a pre-built for 800quid that'll be capable to do what you want with it.
Honestly, building it yourself is the way to go. Take your time, read your manuals. Don't force anything and, if you get stuck, read your manuals again. It's really not that hard.
What exactly did your friend manage to break?
Drop something? Well learn from that, don't drop anything.
Static discharged? Well, learn from that too. Ground yourself.
DOA component? It happens. That's what return periods & RMAs are for.
Where are you located? Chances are you'll find someone (maybe even on here) that'll put it together for you & even show you how to do it for 20quid and some beer (or some other relatively low price point). You really don['t have to pay a shop to do it for you.
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel Xeon E3-1231 V3 3.4GHz Quad-Core Processor (£206.54 @ CCL Computers)
CPU Cooler: CRYORIG C7 40.5 CFM CPU Cooler (£15.48 @ Ebuyer)
Motherboard: Gigabyte GA-Z97P-D3 ATX LGA1150 Motherboard (£64.99 @ Novatech)
Memory: Kingston HyperX Fury Red 16GB (2 x 8GB) DDR3-1600 Memory (£59.92 @ More Computers)
Storage: Samsung 850 EVO-Series 250GB 2.5" Solid State Drive (£55.66 @ Amazon UK)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (£34.98 @ Novatech)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (£34.98 @ Novatech)
Video Card: Asus Radeon R9 380 4GB Video Card (£159.59 @ Overclockers.co.uk)
Case: Thermaltake Versa H22 Window ATX Mid Tower Case (£26.59 @ Aria PC)
Power Supply: XFX 550W 80+ Bronze Certified ATX Power Supply (£51.36 @ CCL Computers)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer (£9.00 @ CCL Computers)
Operating System: Microsoft Windows 10 Home OEM (64-bit) (£72.30 @ CCL Computers)
Total: £791.39
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-12-31 17:32 GMT+0000