Minor change of plan. Housing 2 x 3.5" SATA discs and a 3.5" IDE one is going to be tight. Opted instead for installing a USB riser on the motherboard header, and using a 16Gb flash drive instead. Once booted it should rarely be used so ought to be ok.

I'm using this guide for a PenDrive install (from bootable CD, which I'll disconnect after installation).