Get as much coins as possible and leave the stunts for until you max your car. The more you concentrate on getting the coins the faster will you be able to max your car, and in turn buy other cars too. Start off with the Jeep, upgrade it but don't waste your money maxing it, then buy the big wheels and max it out. Unlock Highway as it's an easy level to earn coins. Unlocking highway means unlocking a pot of gold. You'll never be short on coins after that.