A PSU cannot be too powerful as the computer will only draw the power it requires. You'd be better off getting a new ATX power supply and buying an adapter cable from somewhere like Amigakit to convert all the power connectors you need.
Look on ebay they usually put a picture of the power rating in the listing so you can see what you are getting. It's pretty much standard to get them with 30-38 amp max output on the +5V line which is plenty for an Amiga.
Here is the first one I looked at for example 36A for +5V:
http://www.ebay.co.uk/itm/201449968560
Here is the adapter cable you will need:
http://amigakit.leamancomputing.com/...roducts_id=306
If needed you can get Sata to molex cables for next to nothing on ebay too. Research the good makes and don't buy cheap unbranded rubbish.