OK – then, aside from downloading and installing the game… you won’t need an internet connexion.
The procedure for Diablo II installation is here:
https://us.support.blizzard.com/en/article/000013867
You can download the “downloaders” on any PC and install the game directly on your PC (you’ll need to provide the 26-digits authentication codes during installation) and then copy the C:\Program Files (x86)\Diablo II folder from your install machine to your legacy PC (I think 
).
If you need help for the install – please use the “Legacy Games Tech Support” category here:
Good luck !