If you're planning to use control vinyl with the DJM-S9 you'll need the DVS performance pack.
You don't have to buy the Interface 2 as the S9 already supports Rekordbox. I would also not recommend to get it since there are some issues with it playing vinyl (not control vinyl) properly.
Correct there is no dedicated Rekordbox battle mixer but the S9 would do for that purpose since it supports Rekordbox thanks to an update. What would make the S-9 different to a dedicated Rekordbox mixer?