In your situation the best way to pay is to open LC through bank! And if I were you than if I can spend 20 to 30 thousand US. Than surely I can spend couple of thousand to get visa and go myself to Dubai and check the car personally! But its too risky by paying online because once your payment cashed by the exporter you can't get it back and you never know what you will recieve in your shipment.
But be very careful in these kind of dealing unless if you know the exporter is legit! Ask him if he ever export his vehicules to Pakistan and if he do than ask for the reference!!! If he refuse to gives you the reference than there might be something fishy. ( GOOD LUCK )