If you want KDE 4.10, you'll need to use Linux Mint 14 and follow this post to upgrade to 4.10: viewtopic.php?f=109&t=124912#p684113
. I'd recommend you start with Linux Mint 14 KDE. If you are on another desktop environment and want to add KDE, install mint-meta-kde first and that should probably allow you to select to log in to KDE at the login screen.
Razor-Qt is to KDE what Xfce is to GNOME. So if you need a very lightweight and minimal way to run Qt applications, that might be worth a look.