There are four Horizon outposts, and you get to upgrade them as you progress through the game. Try to upgrade them evenly, instead of concentrating on only one at at time. You are offered cars as you upgrade, and the higher you upgrade, the more expensive the cars are. If you upgrade evenly, you should be able to afford every car. If you concentrate on just one outpost at at time, you will probably be offered a car you can't afford and have to pass on it.