Brand new your target is impossible if you need a monitor, keyboard and mouse. If not then it is do-able although obviously the more you can spend the better it will be overall. That is as close as I can get purchasing brand new parts, maybe try looking for a secondhand 17" CRT monitor, you should be able to pick up a decent(ish) one for £30.
KB-072-LG Logitech Cordless Desktop EX-110 - Retail (KB-072-LG)
£17.99 £17.99
CA-004-AS Asus TA-210 Series Midi Tower (Black) - 360W PSU (CA-004-AS)
£32.00 £32.00
MB-149-AS Asus M2NPV-MX (Socket AM2) PCI-Express DDR2 Motherboard (MB-149-AS)
£44.99 £44.99
CP-162-AM AMD Athlon 64 3000+ 1.80GHz (Socket AM2) - Retail (CP-162-AM)
£44.99 £44.99
HD-023-SA Samsung SpinPoint P HD080HJ 80GB SATA-II 8MB Cache - OEM (HD-023-SA)
£25.95 £25.95
CD-020-SO Sony DDU-1615 DVD-ROM (Black) - OEM (CD-020-SO)
£10.50 £10.50
MY-080-CR Crucial 512MB (2x256MB) DDR2 PC2-4200C4 Dual Channel Kit (CT2KIT3264AA53E) (MY-080-CR)
£33.99 £33.99
MO-006-HY Hyundai ImageQuest Q770S 17'' Perfectly Flat CRT Monitor - Black/Silver (MO-006-HY)
£66.99 £66.99
Subtotal £277.40
Shipping (City Link Parcel Next Day (Delivered Mon-Fri)) £24.95
VAT £52.92
Total £355.27