Firstly is it possible to change the casing (preferably to one with a webcam) and if so where and how much?

I'd be grateful of some help please. It's nigh on impossible even typing this due to the 'cracked' screen.

Try this may be useful

http://www.google.com/search?client=safari&rls=en&q=iMac+17%22+screen+replacement&ie=UTF-8&oe=UTF-8