You can choose to show different content to different regions based on IP and many large sites use it to some extent. One key point from the Google Webmaster guidelines is not not show Google something complete different again from all the other regions. If you do it will be seen as cloaking.
Therefore you should decide which regions version of the site you widh Google to see.
In terms of how much of the content you can change I am not aware of any best practices on this matter so it may be worth reading through the relevant sections on the Google Webmaster guideline pages.
