Hello,
I did a little patch, where you can add a TF sentry gun as a singleplayer enemy. It won't attack monsters, nor another sentry gun.
I did a test map called sentrygun.bsp, so you can run and fight some sentries (it's very short).
Just extract the contents into your quake directory.
Read readme.txt for further info.
EDIT: Do NOT attempt to touch the sentries, or you'll turn into pieces!