I think sea-food restaurant will fill the bill in the City center.
Firstly, you can move this sentence into your first body paragraph.
To add in some words to your intro,tell him why you are writing the letter.
How are you? I hope everything is fine. This letter is in response to the restaurant business proposal you sent me last week. I appreciate the time you spent on doing ample research about the proposal and the opportunity you gave me to do bussiness with you.Having dined at almost every restaurant in the area, I believe the sea food idea will work wonders. Firstly, there aren't any authentic Caribbean Sea food restaurants in the city center. Sea food lovers have to dig down to city's outskirts where one or two restaurants cater Caribbean cuisine.
this is your first body paragraph. expand and give examples if you wish
After much planning and research, I am all in for this idea
This will be your second paragraph,expand and give examples if you wish
In terms of investment, $10000 is bit too much for a new restaurant. I think $7000 would allow us to get started on the right foot.
This is for your last paragraph.tell him why you think 10k is a bit too much to make your letter longer..
there are numerous ways to lengthen your letter, all you need is to have a better structure for your ideas to flow...
hope this helps...