Excessive Oil, White Smoke

I have a 2001 3.0L Privilege. I took it on a long trip and did the usual checks before setting off. The day before starting back it would not start and the indicator on the dash told me to check oil. There was nothing left although I know there was plenty when I checked before. I put in 5L and it started almost immediately. On the way back I took a break and checked the oil but again nothing (below min line). I put another three litres in and carried on. After a short while the engine wasn't running smoothly, the temperature rose to way past it's normal setting, there was white smoke coming out the exhaust and eventually it started losing power. I called in the breakdown truck and they took me to a car hire to get home and the car to a dealership. I had to abandon the car as I had to get home which is over 500 kms away. I have to wait a week before the dealer can carry out diagnostics.

What is likely to be the issue and is it likely to be an expensive repair?