I don't know if others of run into this, but often our scripts are written around "harnessing" existing command-line exe's and gathering input for taking some sort of action.  My "best practice" technique for this is to use the version string (99% of the time any command line program we use responds to "-v" with it's version number) to determine if the file needs to be downloaded or if it existing version is good enough.  

The key to this whole script is hardcoding (and updating) the "CurrentVersion" variable when you have a new version you want to upgrade to :-)

Review, enjoy, and let me know if you have any questions...