How to install KDE in Ubuntu
https://help.ubuntu.com/community/InstallingKDE

If you "like" Gnome, there is no problem just using it, it's basically just personal preference.

Sorry the pdf quality isn't as good, haven't used that myself that way, does it make any difference either selecting the print to file, or selecting the pdf printer instead, are they both about the same? I have used the pdf option in Openoffice a little, it seemed to be decent quality. If you had something important to save "nice", maybe you could save the entire html page, and then open it with Openoffice, then PDF it, and see how the quality is. I don't know, just a thought.

You can set up a firewall if you want to, but if you are behind a router you really don't need to. I used to use Firestarter, I don't think it's being developed anymore, but I think it's still available. You just run it, set it. You don't need to run it after that, the settings are applied at startup. I now use Shorewall, mainly because they are what's included in what I use. I've also used Guarddog (I think).

Linux is not affected by Spyware or Viruses, basically. There are antivirus apps you can install, but you only use them if you are sharing files with a Windows computer. If you download something with a virus in it, Linux will not see the virus, so if you pass that file on, you will be passing the virus too. But it won't harm the Linux computer at all, and unless you move it to your Windows partition, it won't do anything to that either.