New to SEO

Hey all,

I am developing my very first big project and have a couple of basic questions about SEO.

First off; My client is already at the top of Google when searching for certain keywords (canapes & london to be specific). Now I have developed the new site with a new content management system for them ready to be uploaded on to their domain.
My question is, will they remain at the top once the new page is uploaded or will that all be lost then the old site is removed?

and to my second question; The site is built so that the very first page is just a landing page, with three links where you choose which one of the brands three branches you would like to visit (see the site here).
I know that one of the factors for good SEO is the link hierarchy, and so this worries me a bit in the sense that all the three ‘proper’ pages are under /home , /events and /platters
Will this affect the sites place on Google in a negative way? I would hate it if they slip down to second or third position just after the new site is launched!
Is this something I should worry about and if so what can I do to help it?

Hope my questions makes some sense. Thanks for your help

em