Here's a start at the top of your budget, but includes everything. Perhaps you have some bits you could use.
Good, inexpensive, four core, four Hyperthread processor
Decent, inexpensive, motherboard.
Enough memory, fast latency and good speed
Good SSD for OS, applications and some worling files
HDD for slower storage (but still a good, fast HDD)
GPU chosen to a 100 pound budget. Don't know what you really need ATM, maybe even cheaper.
High quality PSU, just right for this system.
Best monitor for remaining budget after OS, ODD, and keyboard/mouse
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel Xeon E3-1231 V3 3.4GHz Quad-Core Processor (£204.68 @ Dabs)
Motherboard: ASRock H97M PRO4 Micro ATX LGA1150 Motherboard (£68.29 @ More Computers)
Memory: Corsair Vengeance 16GB (2 x 8GB) DDR3-1600 Memory (£59.40 @ Amazon UK)
Storage: Samsung 850 EVO-Series 250GB 2.5" Solid State Drive (£58.78 @ Amazon UK)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive (£34.45 @ Amazon UK)
Video Card: XFX Radeon R7 260X 2GB Core Edition Video Card (£95.99 @ Amazon UK)
Case: Fractal Design Core 1100 MicroATX Mini Tower Case (£28.49 @ Novatech)
Power Supply: XFX ProSeries 450W 80+ Bronze Certified ATX Power Supply (£41.99 @ Aria PC)
Optical Drive: Samsung SH-118CB/BEBE DVD/CD Drive (£7.98 @ Ebuyer)
Operating System: Microsoft Windows 8.1 OEM (64-bit) (£70.99 @ Aria PC)
Monitor: BenQ GW2265HM 60Hz 21.5" Monitor (£102.04 @ Amazon UK)
Keyboard: Cooler Master CM Storm Devastator Gaming Bundle Wired Gaming Keyboard w/Optical Mouse (£24.24 @ Amazon UK)
Total: £797.32
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-12-06 17:38 GMT+0000
This is a Legos for Adults project to assemble. My Intermediate School students do it.
http://www.tomshardware.com/forum/id-2625445/build-log-consolidated-middle-school-builds.html
We need more information about your wants and needs and software, to go further.