I had the exact same question when i started my battle visualisation project. I needed thousands of soldiers running around and people kept telling me that Unity cannot handle it. That only Total War engine is capable of this and this engine was built ground up for that purpose.
On the other hand i did not want to start to develop my own engine or borrow an open source solution because then i would be still developing the stuff that Unity already has (and i would probably never finish it) .
Unity has hundreds of developers who work hard every day to make this engine better.
So i decided to use unity and soon enough i found out that almost in every case there are tips, tricks and cheats that will enable to you to achieve the result you are after. I could have thousands of soldiers, i just had to write a smart code that switched some geometry, ai, pathseeking stuff to lower level when camera was further. And now with Unity4.3 beta they have optimized the skinned mesh renderer and i plugged it into my project and have 30 fps with 10k characters.
So yes i am 99% certain that Unity will be suitable for your project. There may be some stuff that are harder to do because you dont have low level access but there are almost always workarounds from this. As a simulation game you really have to simulate stuff that your player sees. The further the object is the less simulation cycles it needs. Also thanks to the popularity of the engine you already get access to modules like flight sim engine. You have already saved hundreds of manhours and thousands of dollars from that. The same applies for GUI system, AI systems,lighting, shaders, visual editors (if you want to use them). There are just too many already done good things for you to lose if you are not going to use unity. And most important is that you get things done.
If still in doubt then just look what v-i-c has achieved with Unity. Hes has a good looking game (which in my mind is not a game but a submarine simulator) that runs on iPhone. If he can achieve that with Unity then you can certainly achieve your vision on powerful computer. You can set the goals "Chris Roberts style" where you are developing your game for a 2 -3 year future gaming computer. And maybe v-i-c can license you his ocean system. It may be not so hardcore as triton but it looks good and has the right feel to it.
As for a team size i cant really say much because i have no experience. My team has a 3 people. 2 Artsist who dont use Unity and me who does code and other stuff. But i know that there are bigger studios 15 - 20 studios who are using Unity. They have their own team workflow pipeline created and by doing some research i am certain you guys manage to work something out.
I also suggest to check this out.
This is a huge project and they are giving very good workflow tips on how to not go crazy
BTW: I dont know if you noticed but i edited my previous post asking about Oculus Rift support. Since your kickstarter will be for he niche market then i think that adding primitive oculus support may help you find more attention outside of the subsim community. I am very active in Oculus community and its full of passionate people who are starving for top quality games that can be played in virtual reality.
EDIT: Since you already have a lot of good quality models may i suggest skyshop plugin.
https://www.assetstore.unity3d.com/#/content/8880 This one is the best lighting solution. You can easily plug it into your game. Even if you wont use it in final game you can still make near photorealistic turntables for ships and distribute these as interactive "thank you's" for your backers