You can definitely do this with one machine well under £450. The biggest thing is size, you want at 2 -4 hard drives and your not going to stuff that in a Mini Itx.
Also don't get so stuck on the CPU Thermal Design Power (TDP) It does not tell you what your system idles at, which in case will probably be about 20 hours a day. Also take that watt usage number from pc-part picker and throw it in the trash. For example: My 4 core xeon with add-in 8 port sas controller and 5 nics, 10 hardrives a printer and a 12 port switch idles at 110 watts and that is without putting my hard drives going into sleep mode.