Hello,
1. you can use entity framework with dotnetnuke, there is one small caveat though as dotnetnuke uses 2 tokens (objectqualifier and databaseowner) when defining database objects so most people use an adapter to handle this -see http://www.adefwebserver.com/dotnetnu... for an example
2. no, it's entirely up to you what's in a skin - when a skin is installed if it's a html skin it contains skin objects that get converted to server control references on parsing - lots of text comes from this and removing the skin object from the html and reparsing it will do what you want (or you can remove the server control from the ascx also) e.g. a skin may have a copyright note you don't want so simply removing the [COPYRIGHT] text from the html will do that. See more on skin objects on the wiki at http://www.dotnetnuke.com/Resources/W...
3. many people (such as me) who have no skinning ability would go to the forge and download one of the free skins and tweak it (http://www.dotnetnuke.com/Community/E...) ,others pay for one at http://www.snowcovered.com/Snowcovere... . Some like to use skinning frameworks such as http://www.thinkofdesign.com/products... that allow them to tweak settings via a UI, and finally some like to use tools such as http://www.artisteer.com/?p=dnn_skins to generate skins and tweak them
4. yes, you can use the community edition - it has no limitations (the professional edition comes with indemnification, unlimited support and a few other extensions). As to customisation, theres a lot you can do within dotnetnuke and it has many extension points to do pretty much anything else - many people get most of the way there with a base install, add a few free modules (http://www.dotnetnuke.com/Community/E... ), buy a module or two (www.snowcovered.com) and have little or no real customisation to do (most commonly you may create a module or modules for something unique to the customer - though in many cases other modules may be sufficent)
5. not complicated, you can download and install a number of free and commerical ecommerce modules - however the modules themselves are pretty comprehensive so setting up a site with all the sku's, variants, taxes etc does take time obviously.
As a general note I'd recommend you take a look at the getting started page on the wiki and have a play around to get an idea of what you can do - http://www.dotnetnuke.com/Resources/W...
Thanks.