Well, will your sister keep her Blackberry? Even if I went for a newer Crackberry, my sister has switched to an iPhone, and the bulk of my family who finally have cell phone coverage (when I went to visit them in late 2006, I got no bars except for one spot in town. The wonders of semi-rural mountain life) are all going for iPhones or cheapy cells. So, I'd have a great phone for email and a PIN, but no one to talk to on messenger.
There are some free chat apps in the Android market. Because I am still on Donut (hopefully not for much longer), there might be more that you can get on Eclair or Froyo that I don't know about. I have Meebo (free) which integrates most IM messengers plus FB or Myspace chat.
Some Android phones do have a physical keypad. I don't know how many are available in the UK, but the Cliq comes to mind. However, I wouldn't bother with the Cliq as there are newer devices.
Besides the mostly free apps, being able to change your battery and upgrade your memory, being able to almost totally customise how your phone's screens look (some models), etc., etc., there's Google web app integration. That means if you use something like Google calender or voice, the corresponding app for your phone updates/works the same.
Here's a forum with a tonne of information on it:
http://androidforums.com/