It is impossible to visit Leshan and Emei Mountain within one day, you should stay one night in Emei and get back in the after the second day. I guess it should costs 500RMb one day (8 hours) to rent an ordinary types car. If you want to rent a van, contact some travel agency for a car with driver. In fact, it is very convenient to take bus. Fot there are direct buses to Leshan and Emeishan from chengdu tourist bus center.