2000 Holden Commodore not sure what it is

Engine Mechanical problem
2000 Holden Commodore 6 cyl Automatic

Hi, I have a commodore vx wagon, automatic. I bought her second hand. (Well third or fourth hand. Was first a police car, then someone used it for a courier service, then it was a second car for a family, and then I bought it)

She is leaking oil from what one of my friends say is the rocker covers. Its two top things on either side of the engine, on one side it has the cap you remove to put oil in. Its from that cover that the leak is happening.

When driving the car a clanking/knocking sound is coming from near the front of the engine, there is a lot of oil around near the oil filter.

I have to put in around near 5 litres of water into the radiator EVERY time I take the car out. If I drive it too long the knocking sound gets louder and faster when I accelerate. And the cars temperature on a hot day, after about 20 mins of driving starts to go up, so I pull over and wait for it to cool down

Water gets pushed out through the overflow bottle. When I stop the car it sounds like a percolator, you can hear water bubbling. Its the water in the overflow bottle. A friend said it could be the head gasket. Another said the rocker covers. I am not blowing any smoke whatsoever. I am not finding any oil in the radiator, and theres no evidence of water in the oil.

Any help would be appreciated, I dont have a tonne of money to replace a head if that is what it is. Please please please help