I use 40:1 which is 125ml to 5 litres of fuel.
as for checking, I would start with the owner, ask him her how long they've had it, any issues, what needs doing to it then check condition and mot's, servicing (who does it) then onto the bike; wheels, ensure spokes all ping etc, then brakes, feel and pad thickness. Frame is it straight , undented ( pressed steel I think, so check for rot near the battery compartment)
then start engine, listen for knocking, leaks etc. then a test ride and see how it performs.
I'd get breakdown cover along with your insurance (usually in with it for bikes)
then if above done, your sorted