A 2 1/2" harddisk up to 4 Gb should work with Workbench 3.0/3.1 (for larger you need 3.5 or 3.9) - should be cheap to get one on ebay, look for second hand laptop harddisks ;-)
You said it had a Motorolla processor, is that in the trapdoor card, or inside the case? If it's on the trapdoor card the number on it is important ...
Oh, and welcome to Amiga.org :-D
