Your have to make sure that PHP is working properly on the Windows servers before installing PowerCMS - You can then go ahead a create the initial site and get the content into the databases. The only thing that you should remember is to name the new site pages a different name from the current website pages - for example, name the current home page index_new.php and leave the current home page named index.php (those are just examples!! You should get the idea) When you are ready to go live, just rename each of the pages to their replacement names and upload those pages to the site - they should be live instantly. You would also be able to check them visually by going to index_new.php or what ever the new page name is.
Using a DW template is no problem - my site uses a DW template and implementation was easy! Feel free to check it out (a link is under my name - just click on it above)......some features I am still working on but it is a working site under PowerCMS.
Modifying PowerCMS to allow multiple users to edit pages and other advanced customizations can be left to do last since they will take more time.....but you would have the basic new site up and running.