IMac 2013 Late 21.5

[JHR]

I have an iMac 2013 Late 21.5. I purchased a new Mac. I stopped the transferring of files because it was downloading too much info and taking too long,,,thought I really don't want all the old stuff on new computer. I would like to backup the old iMac. Is that a good idea if I didn't complete the transfer? Also, there is a lot of info on External SSD with usb to help with the speed on the old iMac. Which External SSD is compatible and highly recommended for the late 2013 iMac, please? I do have My Passport for Mac with 2 TB WD for backup. I am trying to utilize two computers, but using one Apple ID really eats memory / storage for my purposes. NEED HELP PLEASE!