I then re-installed LXDE and Openbox with the following:
sudo apt-get install lxde lxterminal lxnm lxtask lxrandr menu obconf gksu libjpeg-progs miscfiles xli xloadimage
Note, there are some random packages there at the end of that command: not sure if they are necessary, but they seemed useful based on the recommendations from apt-get. I think gksu is a crucial package to help you work in Ubuntu with the sudo security model.
You will be warned about the LXDE PPA security key again.
This process is repeated to make sure any critical packages removed by the previous commands will be replaced.
NB: This will remove Network Manager, so your network connection will need to be reconfigured if you did not change the settings in Network prior to the Pure KDE command. This is easily done - see Reset Network.