Can I purchase a Steam game but install it and play it on a computer without internet?
Using method of moving steam installation to another drive from here you can avoid downloading games but you still have to download steam client:
- Buy and download game to non-gaming pc;
- Copy steam folder to gaming pc;
- Delete everything but SteamApps and Userdata;
- Launch Steam - it will download missing steam files.
This still requires internet connection, but greatly reduces amount of data needed to be downloaded.