315,000 km is about 196,000 miles, definitely past its useful life.  That vehicle is not a good platform for performance and modding it to go faster will only compromise reliability further plus any mods will ripple into the rest of the vehicle’s systems - you can’t just change one thing and leave the rest alone.  And to get any appreciable performance increase won’t come cheap.  I don’t recommend it.

If your car has a 3.5 Duratec V-6, your primary concern should be the internal water pump driven by a timing chain that is very expensive to replace and prone to failure often resulting in catastrophic engine damage. You are past the mileage when problems start to occur. If I had one I would granny it weather the water pump has ever been done or not. A word to the wise.

At nearly 200k miles, I would not even think about trying to add power. The motor is near end of life, and the primary concern would definitely be the water pump on those cars. When they fail, they take the motor with it as @rth3rth3 said.

Just too many miles and not a good platform to start with either. I'd save the money and get another vehicle honestly.

I suspect anything you do to it will blow up your engine and drivetrain in short order, but @mod_man would likely know for sure what's feasible.

No.

I've heard that stickers add about 3hp each.