In running a small business you're probably familiar with small business associations. They'll gladly provide guidance to people just starting out. Then there's the library. Start hitting the books about opening anything food related. Do your homework.
To be honest, it sounds like you're in whoop whoop somewhere. Just because the numbers project some population boom, can you really trust them? Or is it just the hype of real estate venture agencies? (where is this anyway?) (I'm going to Australia so your business plan is safe...
Yes, business proposal. It is crucial for a loan. It will also in itself help you to solidify ideas in your head and yeah, the guestimating is a problem but you have to be realistic. Look at statistics, who lives close, who would be inclined to drive a distance just for the pleasure of eating at your place. Stupid things like gas prices...Yes, it's a Catch-22 - You can't tell how much you'll spend or make until you actually do it, but you can't DO it until you have the money to build it based on telling them how much you expect to spend and make.
Of course lenders are looking for someone who has done their homework. They want you to succeed so you can pay them the exhorbitant lending fee! YOU have to convince them that their investment is a wise one.
Writing a menu is putting the cart before the horse. Having an idea as to the type of cuisine you want to prepare is a good start but you also have to gear it towards the prospective clientelle. (You wouldn't open up a "Honey Glazed Ham" shop in the middle of a Jewish neighborhood would you?)
The final menu will actually be up to the head chef/cook you hire. Someone you don't hire until you have a kitchen for them to cook in. You tell them what you want, they tell you what they can do and how to make it better. <in the perfect world> You can look around the boards or even in your local papers and see what other restaurants are offering as pay for the types of help you need.
We're not even touching the issues about building, kitchen, permits and licenses...zoning...
Like I said: Hit the books. I wouldn't personally ask any of the local cafes for help. It's not like it sounds like you are going to offer the same fare, or that they wouldn't, but they had access to the same info that you do. <probably less when they started>