I recently purchased a EA falcon for $750...only to discover, to my
horror, that the car overheats.
after driving it 200km, I woke up the next morning, only to find it had
a streak of white milky oil on the dipstick when I checked it.

It should be noted, the car recently suffered an LPG gas leak thanks to
the dodgy work of a repco mechanic at cessnock.
Would this cause water to enter the oil?
The motor appears to be running a bit unevenly.

The car also appears to have something wrong with the valve stem seals
or valve guides because blue smoke always comes out on startup. This
wouldnt cause water to enter the oil would it?

If it is a blown head gasket, I was wondering if it would be possible to
drive the car 150km while changing the oil maybe once or twice in the
journey? (ill keep the used oil in a bottle) that way I can drive the
car back to the seller, get a refund and let him deal with it.

Being a gas car, to access the head you need to do things that access
the fuel system like take off the inlet manifold and air filter. this
car has a special gas air filter with some sort of pipe coming out of it
and going from it to the gas converter? possibly gas enters via that
pipe into the injectors or something...dont know though because ive
never really worked on a gas car.

Hopefully I can just change the oil twice over 150km, drive it back to
the seller and get my refund?

-Robbie