If you have to be at HKIA everyday, Sai Kung is probably a bit far.
Driving from Sai Kung to the airport takes about 45min and about 60min in morning traffic (this is based on driving the Shatin route).
As for owning a car:
I won't call it a luxury item per se. Almost everyone in the Sai Kung / Clear Water Bay / South Lantau area has one.
It is fairly cheap to buy any second hand car. Running it can be expensive though.
Fuel: HKD16 per litre. (About HKD120 for Sai Kung to HKIA and back.)
Parking: Average village parking (outside) is HKD1000 per month.
Parking at HKIA: HKD80 per day (there should be cheaper monthly deals though)
License: HKD7000 per year
MOT and service: (depends on model): ~HKD5000 per year.
Toll fees: Sai Kung to HKIA and back: HKD46 per trip.
As for renting in Sai Kung:
You can easily rent a 4 bedroom 2100sqft village house in a nice location and with good finishes for between 50 and 55K per month. (will normally include parking).
However, access to public transport might be limited, so you might need two cars if you use one to go to the airport everyday.