Beamdog has their own client in which you can activate your purchased games through your account there, if you have one.
You can buy NWN:EE from these three major outlets and be guaranteed a working copy:
BeamdogSteamGood Old Games (GoG)You can play Prisoners of the Mist with a copy from any of these retailers. What you have to do is download the custom files and add them to your game folder to be able to access Prisoners of the Mists (and we use quite a lot of custom content here). These can be easily found on the Steam workshop and downloaded, if you buy it through there. Otherwise, they can be found on the forums here and installed manually.
If you buy through Beamdog, you don't have to use their launcher. You can just launch it from the directory, just like Steam, and Good old Games. The client that has the most DRM is most certainly Steam, though, so if you're bothered by that perhaps avoid it.