I am purchasing a new iMac this weekend. Is it possible to use my old iMac, (late 2013 model), as a second screen? If it is possible what cables, ect? Thankyou in advance
iMac 21.5″, macOS 10.15
No. While Late 2013 iMacs running High Sierra or earlier can act as Target Display Mode displays for other old Macs, Apple placed restrictions on "the other Mac".
A new iMac would be too new to act as "the other Mac".
Use your iMac as a display with target display mode - Apple Support
+1 No, Macs are computers they are not external displays for other devices.
If your new Mac requires a 2nd display, please buy one. This solution is the most reliable, easiest to install and less expensive as it does not require expensive cables. There are many many many external displays to select from that fit all budgets.
Using 2013 iMac as second screen for a 2nd screenfor New iMac
