Steps to ensuring data integrity / security of Mac mini prior to sale? I've got some Mac minis ranging from 2011 models to 2014 models, which I wish to sell. My concern is with the integrity / security of the data on them. 

I don't have a large external HD / server to dump all the data onto, so my plan was to take out and keep the HDDs from each Mac mini, and put in a brand new SSD prior to sale. 

Other than doing the above, is there any steps I should taken to ensure integrity / security of the data? E.g. will anything be saved to the RAM or CPU / GPU or other components?
RAM is volatile, meaning it loses its data when powered off.   

The same for the other components, and they don't really 'store' data, and it's likely to be in a form that doesn't lend itself to useful 'human' level data.

If you're wiping or replacing the storage device with a new/clean volume, then that won't contain any info associated with your AppleID. However, you'll need to remove the Mac from your ID's list of known (and therefore trusted) devices.

All the serial number records are tied at Apple - and Apple is designed to let a new owner “claim” that serial number but not snitch on who had it last. There’s never any reason for Apple to disclose who had gear other than a legal proceeding - they just register the gear to the new owner and that’s who “owns” that serial for now.

You could just perform a Secure Erase on the disks, though at their age that might be the final straw.