Good choice in downloading and installing EB! I will try to help you with a few of your questions.

1. There are two types of military buildings you can build factional and regional. For example, if you are playing as the Romans, your factional military buildings would produce your hastati, principes and triarii etc. Your regional military buildings would produce local troops, such as Ligurian infantry and Samnite spearmen etc etc.

2. I don't think it's that different from Vanilla R:TW. Some things like mining centres and improved docks are expensive, but this just symbolizes the rarities of said buildings and how much importance they hold.

3. EB isn't like M:TW 2 where you have military cities and trade cities. Although, you can just concentrate on building trade cities by building farms and ports and such instead of building military buildings.

4. No, it doesn't. The sea is no different from the vanilla sea in the area that matters, the size. It does not lag your game.