The buildings that you need are the Training Camp and Vehicile Factory. Once you have these you will have numorouse options.
The gunnar is a cheap unit for 450 coins. The gunnar will shoot rounds with an assault rifle. It has normal range and low health.
One Army Unit
The sniper isn't so cheap going for 1,200 coins. It has high range with a slow fire rate and low health and speed.
One Army Unit
This is the most expensive unit for 2,500 coins. It sends out a rocket attack with a slow fire rate but just as powerful as a tank with splash damage. The one downside is it does not have as much health as the tank however it can tank an explosion itself.
One Army Unit
The Humvee is a fast vehicle that can pick up troops and has a mounted turret on the top to shoot any nearby threats.
One Army Unit
The Light Tank is a cheap way to get a unit with a lot of health and splash damage attacks. Being not as expensive and costing less units it is better for early game when you don't have enough money for heavy tanks and houses.
Two Army Units
The Heavy tank is the best unit by far. Using the rocketeers explosive rounds but improving massively on it's health, the only downside is the amount of army units it uses and the cost. Being the second most expensive army unit it is incredibly worth the price.
Two Army Units.
The Missile, the most expensive army unit, is very effiecient. It has one of the biggest disadvantages being it can't attack units. But it's increbidle air attack and same range as the Core it is worth the price.
Three Army Units.