No need to buy the radio.
From my 1 year experience as Leo all i use are these
10-4 (okay/message received/understood)
10-5 (requesting additional) if you need dot you would say 10-5 DoT to [location] most of the time when LEOs request additional LEOs they just use R/A)
10-8 (on duty)
10-10 (off duty) (some would say 10-10T if they are going off duty temporarily) (I personal don’t use 10-8 or 10-10 because I find it pointless to tell others I’m on team)
10-20 (location)
Code 3 (responding with lights and sirens)
Code 7 (pursuit in progress)
These are what I personally use because I’m not a big Leo enthusiast who takes this game seriously enough to be professional.
But here is a forum post with every single code