rebuild engine

You might check out Kelly Bluebook or some other source of used car values. If you discover that that car in factory mint condition is worth significantly less than the cost (including labor) of replacing the engine you might opt for a new car.

What you have already put into the old one should not affect your decision.

That's the hard part, and I agree. I think it's a common misconception to think that putting x dollars into something makes it worth x dollars more.

With all of that in mind, I don't see a scenario where where replacing an engine in a 15 year old Toyota would make sense.