As I have two servers, where would I best look at selling one of the servers?

ebay

# The servers come with a VT420 console, is there a GUI for SCO that I could use via my Win PC?

youd probably have to run x, then find some program that will run an X session on windows.

# Could I install a version of linux on this type of server?

linux runs on a wide variety of architectures, you just gotta find a distro the runs on your arch.