ok ta mate said the same, so ordered throttle cable, throttle grip, and might as well change the engine oil (ensure previous owner did it)
If I empty the oil and it looks brand spanking new, is it ok to put it back? (filter it with some kitchen towel)
Yes. But as you don't know what oil it is, how old it is or even the viscosity, replace it for peace of mind.
It's a 125, it'll probably only have four teaspoons of oil in it anyway.

guess problem is some people run bikes to the ground (I know that myself someone who used to be mechanic in shop) then comes so expensive to replace everything cheaper to buy a new motorbike.