Hey Dan,
If you are correctly using rel="alternate" hreflang= meta tags or sitemaps, then it shouldn't matter if you have mostly similar content. One of Google's examples is just like your question:
Your pages have broadly similar content within a single language, but the content has small regional variations. For example, you might have English-language content targeted at readers in the US, GB, and Ireland. Now, your question on IP sniffing is a slightly different kettle of fish. Google contradict themselves in their own guidlines!
On this page, Multi-regional and multilingual sites, they say:
Avoid automatic redirection based on the user’s perceived language. These redirections could prevent users (and search engines) from viewing all the versions of your site. Yet here, Introducing "x-default hreflang" for international landing pages, they say:
The homepages of multinational and multilingual websites are sometimes configured to point visitors to localized pages, either via redirects or by changing the content to reflect the user’s language. And then there is this recent video from Matt Cutts: Is redirecting users based on their location spam?