Hi, and welcome to LQ!
To check for and install missing packages, open the terminal and type:
sudo apt-get install updates
As for multi-boot systems, you can have as many as your HDD can hold. Each new install will check for installed systems and configure Grub to include them in a menu at startup. Just make sure, when presented with the option, that you allow enough space for your current system(s) to "grow". I'm not personally aware of any that have a problem with multi-booting. I'm also not aware of any disadvantages to using an external drive.
Hope this helps