One thing you could do is flatten the hill completely and then in the middle curve it downwards into a bowl shape to increase potential energy. That would be because you are going downwards, and when you are going downwards you collect more kinetic energy and when you go down a slope into a curved bowl then there would be less kinetic energy. The car's descent into the curved bowl would make it slower since its not a perfect slope and there would be more traction.
I would recommend that you create smaller hills in a row but doesn't affect the overall length of the hill since a car traveling up high elevations multiple times in a row would make it increase in potential energy. The short lengths of the hill would prevent the car from gaining much kinetic energy.
The best answer would be 750kg since making the car heavier would help it to increase gravitational potential energy. As a heavier object moves up a slope, it gains more potential energy.
50m would not be useful since it affects the length of the hill and not the height of the hill.
Making the hill taller would not help since that would help it to increase kinetic energy.