I have an iMac, which consists of a screen and a keyboard (no 'lump', no tower, as all the gear is contained in the back of the screen which weighs about 5kg).
Brilliant machine.
Apple service is excellent. I'd had the iMac a week and couldn't get internet, so they sent out a second brand-new iMac to replace it (and as it turned out, it was ntl who were at fault, not the Mac).
In two years, it's gone wrong once. They picked it up next day, fixed it, delivered it back to me on day three. (I do have a maintenance contract with Apple).
I haven't found a website that doesn't work with Mac OSX. The only program I've found that doesn't go is Internet Explorer (+Outlook), and that's because Microsoft stopped making OSX compatible versions. But Firefox/Safari/etc work perfectly, and the built-in Mail program on OSX is problem-free.
Yes, more expensive to buy, but maybe not in the long term. (Sorry to ignore your question re. using existing screen).
Allen, London.