I have a Netgear ProSecure UTM50 (the Netgear ProSecure line is end of life.)
Is it possible to install something such as Sophos UTM/Untangle/etc. on it?
Yes, you will be able to.
Just make sure the software is compatible with your hardware specification (NICs, CPU, minimal disk and RAM size) and that it’s not hardware locked. (for example Sophos offers a Software and Hardware version, you’ll need the software version for non Sophos products).