A purchase is for 1 site/project only. There isn't a real difference for "internal" projects or "external" (so those live or not live, a project is a project). It's different of course if you develop on a local install, use it there and use it on the live project once this local install goes live.

As for downloading, yes, you can download paid Add-Ons from the marketplace (once purchased). You're not allowed though to use it on more then just 1 project (although stated otherwise by the developer). Of course, people can use the download button to their advantage and many will be doing that. That's the disavantage of technology and applications that are bought/downloaded from the internet. But by downloading the appropiate licenses you will support the CMS (30% of the price will go to this site and their developers) and the developer (the remaining 70%) and make sure it will maintain up-to-date and gets updates.