This is how to create an army fully based on just gaining money. Note that this military will not be strong but just there for earning money. First, keep the units you already own with you. Then start buying various income buildings. Here is just a small scratch note of how many of each income buildings you would need to buy in order to have a high income: 101 Supply Depots, 77 Refineries, 51 Weapons Factories, 45 Power Plants, 17 Oil Rigs, 5 Military Labs and 4 Nuclear Testing Facility atleast. This is how much you should have, but if you do not have the specific income building unlocked yet, then buy the amount of each of every income building you do have. 
#             Building                       Total Income of Building
101          Supply Depots               101,000
77           Refineries                     500,500
51           Weapons Factories         841,500
45           Power Plants                 1.792,000
17           Oil Rigs                        4,590,000
5            Military Labs                  2,500,000
4            Nuclear Testing              2,800,000
                                                                                 
Total Amount of Money Made: 13,125,000


While you are creating this high grossing income, also make sure to buy many units. Atleast 10 of each type of unit. It is not a killer army, but good enough.