I am building a new website for myself and have toying with the idea of implementing a CMS with it. How difficult would it be to adapt my site to work with SilverStripe? Adding new menu items will never be an issue so it will always just have the 5 menu items.
Ironically, adding new menu items is probably one of the things a CMS excels at! In SS, you just have to create a new top-level page and the link appears in menus automatically.
To get the exact same look as your current website, you'll have to create a theme, and perhaps templates for the different pages. If you browse the tutorials, you can find more information on how to do this. If you're happy to use the default theme, then it's very easy: you create your 5 pages and copy-paste the content into SS.
Have you thought about why you want to port your site to a CMS?