Engine questions
I don't know what unity costs, but it might be overkill. I think realistically what would be the best option would depend on what this game will be.
If it's a FPS, then to me the big appeal is being able to have 200 guys charge out of a trench with fixed bayonets. I'd maybe suggest the mount and blade engine in that case since it doesn't have much trouble doing this when those bayonets are on muskets rather than rifles. Not pretty stuff, but realistically, how many of you besides myself have ridiculous AAA gaming rigs*? When you have that much shit going on (gas and horses and mortars and machine guns and CHARGE! and stuff), do you really pay much attention to how things look? They have also made everything from a standalone GS/FPS pirate game to a marching band simulator out of this.
If it's an RTS, then I don't really care about how it looks or amount of people in MP as long as there is no unit cap beyond what you can produce.
A grand strategy/RTS hybrid has always struck me as something WWI would be suited to - this is why I suggested modding Xenonauts. Unfortunately, I don't think whatever they built that on is at all capable of MP support, and is probably proprietary. Might be worth looking at whatever Running With Rifles is built on in this case - engine has MP and strategic elements could be introduced.
*Don't buy one. It's a waste of money when 90% you play is ascii games or games built to run on Pentium 3s.