Here's my suggestion, buy new HDD and scrounge the rest used.
My current webserver is a 800Mhz P3, 512Mb SDRAM, random stock intel micro-ATX MB, no-name cheapo CD-ROM drive, and the most expensive part of the kit, a 120 GB HDD.
I have a newer 1 GHz P3, and some ram I have been meaning to...