Internal SCSI on a Mac Plus has more than one meaning. Manufacturers produced alternative SCSI interfaces (ROM or CPU clip-on boards) which were better than the built-in. Some were used in conjunction with internal hard disk drives.

How much power? If you have refurbished the PSU and fit a low power hard disk drive, there is nothing to worry about a SCSI extended Mac Plus.

Edit: Don't run the Mac Plus 24/7 until you have calculated the risks.