You don’t need to purchase anything.
The main problem with playing players vs bots is the fact that you cannot uneven the teams. Meaning your friends must join one by one as you add another bot. Use admin addbots 1. You’ll also need to be able to log in using adminlogin PASSWORD, and have to set up said password in udkgame.ini.
You also cannot stack the enemy team with more bots because using AddRedBots/AddBlueBots results in an infinite loop and immediately crashes the server, bots were obviously not a terribly high priority.
Yikes, so if it’s just one friend and me, we’d be stuck playing 2v2 the entire time?