Engine Oil Pressure Low and Drivetrain issues?

My father's car is listed above and has over 100,000 miles. For a while the drivetrain issue was saying drivetrain malfunction and my father has been ignoring it saying that it's fine. I told him to get it checked out and at BMW they want to charge from $800-$1,000 to fix it, and my father was reluctant. A few days ago, the car showed me a message "Engine Oil Pressure Low. Stop Carefully" and something about further damaging the car. I stop off to the side and check to see what's wrong. I called my father and he just said to add more engine oil and I did. The message didn't go away and when we took it to the repair shop, they wanted to charge us $6,000 for a new engine and then another $1,000 for a new drive train. This is basically the same price as the lease for the car. Is it worth it to get that new engine for $6,000 or should we go for a use engine and ask a local mechanic to put it in?