The torque required of your steering servo depends on many factors besides the size and weight of your car - wheels used, terrain, mechanical design of the linkage, required steering speed, etc. For a 60 kg car, it will have to be quite large - bigger than a typical hobby servo. Probably a large DC gearmotor with encoder feedback and a separate servo controller board. Without knowing all the parameters mentioned above, my gut tells me you will need something like a wheelchair motor or similar size.