Valve adjusting and an oil change won't hurt..! So you can keep track on maintenance schedule of the bike without asking the PO when he did the last oil change or valve adjustment..!
lil backfire on a low gear on downhill sounds like mine..! Mine does that too..! I can only hear hear it if i concentrate very carefully..! even then i'm not sure it's really backfiring..!
I've ridden it nearly 4000 km after i got it and so far no issues..!