If you use Mech's reader or an SSD, you won't need much power at all. I guess you could jump a couple of lines from the PSU in on the A500 to the SCSI controller, but put in a quick connect-disconnect junction so you can move or work on it. The GVP Guru ROM is a download from Ralph Babel's website, but the adapter is the bitch to locate without getting the whole GVP Impact Series II card; except they are hard to find cheap (being rare). If the SCSI controller is a WD 33C93 DIP series and not a rev 8, then get an AMD from Mech for 15 bucks. They are more stable with CDROMS and other devices. The Guru ROM adds no more than 0.5 MB/s to the transfer speed over the stock ROM and even that is only on certain transfers -- not worth the money to find and buy one. Now you do have the option on an internal HDD and hooking up the SCSI card reader in an external SCSI box (I got all of mine searching for Mac's external SCSI drives). I've done this with good results but it will need termination. The transfer rate for SCSI/2 is rate limiting on most drives much less SSD. to stay in budget I'd avoid the 100 buck SCSI->IDE/SATA and stick with CF cards, CDROMS, and hard drive from decommissioned servers.
The $60 SCSI card reader will give you a lot of use; I plug in a 4 GB card filled full of WHDLoads and it is retro play time, or use another one with bootable OS or backup.
