Do you want to make a game about corporations, colonies, planetary or regional governments, etc.and their economical incentives? If such entities are autonomous, they are natural "enemies" of the empire, to be manipulated and kept happy; if they are player-controlled, the player can only control one of them coherently, not the empire and not multiple ones. What do you have in mind?
I'm thinking of a game where an empire consists of multiple entities (populations, corporations, government) with the player controlling the gov and AI (or a set of equations) controlling the rest. Keeping an empire alive should be a balance between the different entities. This balance can be upset by trade imbalance or propaganda.
But if you are at war with them, how can your trade with them? Each empire builds ships within the limits of the resources of their own planets and stations, possibly buying finished starships or appropriate resources from the same third parties in exchange for non-starship goods and resources.
Wars are not started before the military is ready, at least on one side. As one side is building up, the rest of the world can take actions. An empire can keep secrets but only for so long and only until an operation reaches critical mass. If Russia ramps up military production, the rest of the world will know before too long. This ramp up may take a year or two, in the mean time trade sanctions can be made to slow them down.
