Hi, I am also looking to replace the tap with a ball valve.
I bought a 1/2" ball valve http://bryggeland.no/kuleventil-3-deler-1-2-tom.html
I have measured the female thread on the Braumeister at 1" (25mm) so I am going to need a stainless 1" male to 1/2" male reducing nipple and some PTFE...