Sorry I'm confused, avast should have no problem installing on a stand alone system not connected to the internet.
What is this, 'I suppose Avast will not install properly at this stage because of the missing connectivity' what problem ?
The only problem would be it can't connect for updates, you would have to do that manually by periodically downloading the VPS file and installing it on the stand alone system, the same is true if there are any program updates you would have to download the complete program and install over the top of the existing version.
Because of this being a stand alone system with no internet connection you would have to change the Program Settings, Connections (Basic) to Manual for both Viris database (VPS) and Program, otherwise avast will check to se if a connection is present (it doesn't try to establish a connection).