Amigakit is one of, if not THE best place for this kind of conversion.
Here's a brief list of the essential things to buy first:
EZ Z4 Tower - product code: EZTOWERZ4
ATX to Amiga 1200 Power Adapter - product code: ATX2A1200
Tower Case LED Adapter for A1200 - product code: ADALEDA12
EZKEY-SE Amiga Keyboard Adapter - product code: ADAKEY005
4-Device Buffered IDE Interface (A1200/A600) - product code: ADAEZCDSE
2.5" HARD DRIVE 44-WAY CABLE (20CM) - product code: CABIDE044
Amiga Floppy Cable (40CM / 34-pin) - product code: CABFDD040
Any spare CD drive will fit perfectly fine inside the tower.
