The Win 10 activation will only be applicable to the machine you updated. The license is not transferable to any other machine. You would need to buy a new license.
The iso can be downloaded from several sources but that would not do you any good as it would not activate if installed on another machine.
MS are giving you a free UPDATE for an existing machine not a free install of Win 10 to anything you may buy in the future after the free offer ends next July.
If you want to install it on another machine then if iot already has Win7 or * on it it will be updated as you would already have a valid license for it.