[edited]
Ooops, Matt comes before me.
An ST506 hd don't carries too much data, better donate it to junkyard or a museum.
For better use of your A2k, buy an IDE HD (cheapest way), but you need a Zorro II IDE controller (none peecee would work).
But probrably you only find a SCSI controller. In this case you need a SCSI HD (or even an accelerator board WITH a SCSI HD).
Look at ebay for A2000 accelerators. They are not expensive (if the processor is <68040), thought. GVP Combos are the best for the A2k, IMHO.