In general, its good that you've narrowed down your use case.  This will be helpful to you and is, unfortunately, a step that some folks tend to gloss over.  Shopping cart integration with a CMS (IMHO) is a more demanding exercise on the front end of a project, but you're wisely asking about TCO which is where it pays off.  Having said that, you'll need to ensure that you're comfortable with the shopping cart extension that you use.

DotNetNuke excels in the infrastructure categories but does bring some complexities that are inherent in the MS stack.  There are lots of configuration options and it can require some patience to work through optimization.  But your finished product will have all the scalability and security capabilty that you hope for.  The .Net platform is far more secure than LAMP (you can Google about that for yourself) and our security team is highly proactive.  You should take a quick look at our history of dealing with (the few) security issues we receive.

The platform is engineered from the ground up for extensibility, which means that integrating other systems is limited only by your imagination and development skill.  DotNetNuke is used in all kinds of applications from front-ending SAP to online banking, etc. not just community oriented sites.  Support for localization is also built into the framework and we have an International Community Team working alongside our engineering group now to further enhance capabilities for managing multi-language content (there are many solutions available).

Hope this is helpful to you.  And good luck in your endeavors.

Scott Willhite, Co-Founder DNN