Can that be done ??
What the auto-power or the CF bit?
Auto-power is easy and as long as you have a solid core PSU then it will be fine, but using a CF card will draw a lot less power than a hard drive.
I have a A590 which i think has a 120MB HD ... Its SCSI , so what would i need specificly to put a CF instead ?
You need a
SCSI->IDE adapter, an
IDE->CF adapter, a CF card and a USB CF card reader.
I would advise making the card removable (i.e. cut a slot in your A590 case) it will make it sooooo much easier to transfer files from the PC to the Amiga. Believe me it's like having a new Amiga being able to transfer data like that.
Of course you do not have to make it internal, with an old SCSI external enclosure (like one for a CD) you can add the SCSI->IDE->CF in place of the CD drive and then connect via cable to the back of the Amiga.