Probably the minimum you need to get in is about 30% of what you can read on here about OASC. However, I would recommend aiming to be at at least 70% to give yourself a good chance, the competition for places can be very high indeed. You don't have to do the Aircrew aptitude and medical anymore.
I would caution being too positive about your PPL, this used to be seen as a bad thing. Although the UAS system has changed recently, and flying training is no longer their focus, there may still be a lingering history/feeling that PPL people are a pain in the b*tt because they think they know stuff, but they don't know it to the RAF standard or style. Stick with presenting it as 'keen to fly' or 'really made an effort to see what flying is about' rather than 'nearly got my PPL'.
The actual process is mentioned on a thread that should still be on the front page here.