I'm like the previous posters, the tanks are killing you. What I would do, is invest in a few factories; not only do they greatly reduce your infrastructure costs, they also make tanks and cm's alot cheaper, so they are easier to purchase when you DO need them.
A really good trade set up for infra buying is this:
Water, Wheat, Aluminum, Lumber, Marble, Iron, Fish, Cattle, Sugar, Pigs, Spices, Gems
Bonus: Beer, Construction, Fast Food
The gems can be replaced with something else if necessary, but this is a fairly common set up used to lower infra costs.
And by the way, I would STRONGLY recommend picking a team color. There are "grey team" nations, but they are usually older nations with political reasons for doing so. Even if you do have political reasons for being on the "grey team" at your NS level, it would be advisable for you to pick a color and pick trades which match so you can get the team bonus.