Honestly if this is all within the company I would recommend against Dreamweaver. While I primarily code in PHP I have started to use Sharepoint services more and more. The Sharepoint services will allow you to setup interoffice communication much easier than developing in Dreamweaver would.
As far as what to install, if you have IIS you should already support ASP and .NET which version only you know. Dreamweaver can program in ASP by hand, but support was dropped as of CS4 (v10) because Microsoft was releasing the code to Adobe after they the market so Adobe stopped playing catchup and focused primarily on PHP. I installed a copy of PHP on an IIS 6.0 box awhile back although now everything has moved to IIS7. Typically for PHP setup you could follow these instructions ( http://learn.iis.net/page.aspx/247/using-fastcgi-to-host-php-applications-on-iis-60/ ). Fairly straightforward for IIS 6. The FastCGI method tends to perform better.
If you want to quantify what can do more, PHP does more with Apache then it does with IIS and on the other end of the spectrum ASP and .NET only really work on the IIS platform so they are able to take full advantage of the system.