You can install Lightroom Classic and other Creative Cloud apps on up to two computers. If you want to install it on a third computer, you'll need to deactivate it on one of your previous machines. Learn more about activating and deactivating Creative Cloud apps.
Yes, possibly. 1:1 previews can take a significant amount of time to create and take a lot of disk space. Read this article to learn how to optimize your use of previews and get other tips for optimizing performance.
In the Develop module, Lightroom Classic uses the ProPhoto RGB color space. In other modules, it uses the Adobe RGB color space. Learn more about color spaces, color profiles, and other color topics in this FAQ about color in Lightroom.