DCS World 1.5 and 2.0 are separate installations. You should be able to get both installers from the DCS World website.
Nevada is basically the only terrain in version 2, which means it needs to be purchased to even be able to use version 2.
The developers are currently in the process of combining the separate versions, which will mean that the Caucuses (an improved version) will be available to use in Version 2, along with all of the 1.5 aircraft, missions etc.
Here is the installer for V1.5
http://www.digitalcombatsimulator.com/en/downloads/world/stable/
You can have V1.5 and V2 installed on the same system/drive. I actually have those two and V1.5 Beta still installed also.
Cheers,