BBM
I think we're not well placed to know right now.
My personal feeling is that he's committed sexual offences before that may or may not come to light/be in the process of investigation already. I don't feel he's murdered before as tbh I feel he got so overexcited about murdering Sarah that that was when he started making stupid mistakes. I also think, as I said upthread, that he's more likely to have hired a car in his own name for anonymous encounters with sex workers than with the expressed intention and premeditation to murder.
However, I know for sure that LE were looking at other potentially connected unsolved cases within a geographical radius once they knew who had murdered Sarah, as they contacted another Websleuther for more information about something that came up here, and, if people recall, there was a slew of other unsolved sexual offences and disappearances of women in the Kent area that were discussed on the threads at the time as possibly being linked.
I don't think we would necessarily be hearing publicly about what stage any of those other investigations had reached until LE were ready to bring charges and iirc there were way too many of them (!) for the media to have much of a handle on which ones to pursue. So, on balance, although I don't think I believe he's murdered before, I also don't think the lack of anything else in the public domain atm tells us much one way or the other.
JMO