You know, I'm leaning towards not fixing what's not broken. It might not be hip or sexy, but for this application SSI has much to recommend it.

• Zero learning curve
• already installed, ready to go, proven, stable
• No more resource hungry than a templating system
• While it does use resources each page serve, this is a low-traffic site on a private server that won't even notice the minor load
• Although it is static, the content and the nav bar do change from time to time. An off-line generator would mean having to re-generate the entire website each time any page was updated, adding steps of complexity in a a situation where pages may be edited by multiple authors who have limited web skills to being with. Or I have to develop an interface for that function too. Whereas with SSI, edit the file directly once, and that's it, done.
• same for the nav bar, which changes occasionally;