What should i choose to attack with ?

Either! As long as no reinforcements come up or he gives his units CAS, you should be fine.

A lot of mech units are very useful against infantry. However, you could also use overpowering force to outweigh casualties.

I would go with the 2 Armored Cars + 1 Light Tank.

Cars suck, they are not worth building.

I never build cars no more. In my first couple of games I used to make them while you could still have them before they would need lvl 2. But usually I focus on infantry, tanks and planes.