Finish the singleplayer first before buying a car yourself. Since you get random cars by the time you finish SP you will have about 60 cars given to you for free. So dont waste cash on something you might get for free...then you just spent money you cant grt back
Also by playing SP you will be getting tons of cash and xp anyway. Obviously afterwards the money glitch is great because Sport races dont give as much or as frequent cash n XP.