I would assume that such an email client would have half-decent IMAP support. Besides copying your contacts from gmail, this should do everything you require.
The problem is that you don't want a copy, you want a maintained copy. You want your data synchronized.
Outlook does this very neatly with Exchange, which is why it rules the enterprise. Android does this very neatly with your Gmail/Google account. The iPhone also does this to some extent (at least against Exchange contacts).
Once you get used to the idea that there is only one set of data, going back to working on different sets, different copies of what should be the same data feels very, very wrong.
You can say what you like, but a copy alone will not do "everything you require" for most people these days.
1. http://mail.google.com/support/bin/answer.py?answer=75725
FWIW the last time I tried KMail with GMail, there were some issues with how the folder-to-tag mapping worked. This ended up with me needing to untag a lot of stuff manually in my GMail. But since this was over a year and a half back, I wont really go by this. Things could well be better now