What do you need the ATX PSU or adapter cable for?
Not sure you can use AmigaOS3.1 with KS3.0 ROMS.
The cheapest way to get an A1200 if you are in the states is to do a deal with a UK EAB member. You buy a UK A1200 from eBay for $50 and he ships it to you. You do the same thing for him for something only available in the States.