I recommend you first upgrade your face to max level, which gives you a 40% discount on all cars and clothing.
There are four car dealerships, one each in Central and Kennedy Town, two in Northpoint;
Central Impound Lot - In Sui Shan Alley, Central. It's Directly East of Soho, and South of Wah Fi Storage. There are six cars to purchase, totalling HK$1,248,000
Kennedy Town Docks - At the westernmost point of the map, on Kennedy Docks. There are seven cars to purchase, totalling HK$3,255,700
North Point Chop Shop - Near the bottom of Northpoint, East of the Hang Sui Clothing Store above Yau Ling Park. There are six cars to purchase, totalling HK$343,400
Top Glamour Imports - On the road just north of the entrance to the shipyard in Northpoint east. There are seven cars to purchase, totalling HK$1,909,100
Altogether, it costs HK$6,756,200. With the Dai Lo Upgrade it costs HK$4,053,720. If you think you've missed one, enter the Statistics Menu, then Assets, then Vehicles. It should tell you which cars you are yet to own.