its a standard PC power supply, 300watt? not that it matters really as the PSU is only driving the motherboard and either a HD, or lower powered SSD.
£20-£30 should be enough. check the connectors on the HD. if its SATA and not the old PATA, then make sure your new PSU has SATA connections, though said that, SATA has been out for many years and probably standard on the smallest of PSU's
the PSU going i would say is the biggest contributor of on/off not working, and BSOD's.
let us know buddy, even take a video of your process so we can see whats what.Go to the full post