how hard would it be to fit a windsor out of an au xr8 in to my au xr6 ? and rough how much would it be for a used engine with low kms?

I'm not sure about what you should expect to pay on average for the engine.

It shouldn't be too hard to do the conversion, do you also have access to the looms, computers etc?

I'm not sure if this applies to the A series or the XR's, but if you want to make it legal you'll also need to swap over the front brakes, booster and springs. Which are obviously to help brake / spring the extra weight of the engine, but as I said, I'm not sure if the XR's have the upgraded brakes standard and if the 6 and 8 cylinder systems are the same.