Will Kubuntu 10.04 be an LTS

http://www.kubuntu.org/news/10.04-lts-beta-1

Yes it will be an LTS.

You can also run KDE applications from Ubuntu. They run fine. The only problem I have had is that sometimes the Help documents are missing, but that's rarely a problem, and they are generally available online.

Hi :)

Yes, it is an LTS release and that was determined ages ago. It's worth checking their home-page
http://www.kubuntu.org/

Yes, you can install Kubuntu into an existing Ubuntu install "just" by

sudo apt-get install kubuntu-desktop

You might need to logout and use

Options - Change Session - KDE

But this will mean you have installed 2 of the heaviest linux desktop environments onto a single install. Much better would be to make a cd of Kubuntu and try it as a LiveCd. It's always a good idea to try an upgrade or a new distro as a LiveCd before committing yourself to it.
https://help.ubuntu.com/community/LiveCD
It runs a bit slower because it's running from the Cd/dvd-drive which is a lot slower than a hard-drive but it lets you get a good demo of what its like and how well it works on your hardware.

Good luck and regards from
Tom :)

Thanks to you all for answering my question well and for all the detailed advice Tom gives. I think the idea of trying a live CD for a little while makes lots of sense.

all the best,
Joel