Some photos of my build. Need to add in another 8Gb RAM and fit handle and power button, but final spec will be:
M69 case + PCI-E riser + 450W PSU = £120 approx after all parts, shipping and tax
Asus Prime H310i-Plus R2.0 = £40 (2nd hand, Facebook Marketplace)
Intel i5-8400 (Hex core) = £92 (2nd hand, eBay)
Noctua NH-L9i = £35 (new)
RAM = 16Gb DDR4 Corsair LPX 3000 = £47 (2nd hand £15 + £32 new)
GPU = AMD R9 Nano 4Gb = £80 (2nd hand)
SSD = SK Hynix 256Gb NVME= £30 (2nd hand)
Total cost of £444
Running Prime 95 and MSI Kombuster concurrently for 10 minutes with no additional case fans, the CPU reached 65C and the GPU 75C. This is both at 100% load. PC was audible but acceptable for such a small system. At rest it emits audible noise, but it's acceptable. I will record some videos tomorrow and post so you can hear the noise levels for yourself
This has been the easiest PC I've ever built. I was gaming inbetween building, but i reckon I could have had this fully assembled and booted in around 35-40 minutes.