I stayed at the San Cristobal 2 times. First stay was upgraded to executive suite and on second stay was upgraded to presidential suite. both stays were for 7 nights. they treat platinum members very well. you have access to the lounge for breakfast. you also can have breakfast at the hotel restaurant.
regarding transportation from the airport, i have asked the hotel to pick me up from the air port. they charged me about $US 35.00 i also saw a sign in the airport for shuttle service, i think it is $US 10.00
yes, there is a budget car rental at the sheraton hotel ( right next door). i rented a car from them for one day. they are very expensive. a small car was for $110/ day.
make your car reservation on line it is much cheaper.
Enjoy Chile, nice country and friendly people.