It is always safe and secure to book your car for transportation with the agent who book your hotels and other tours. Because their drivers are well trained, polite and informative.
Yes you get cheaper options from airport. But need to worry about the quality of the car and trust worthiness of the driver. Most local drivers are not informative about the hotel locations. For instance, most good accommodations in Munnar are 30 minutes to 1 hour away from Munnar town. For extra distance run to reach your hotel, they charge the way they like. Most tourist say, like this way, they waste more that 1 hour searching for the hotel.
Vehicles available for travelling are Ambassador, Ford Ikon, Ceilo, Tata Sumo, Toyota Quallis and Large Coaches etc.