Personally I feel that Avast One (Essential) doesn't give the same ability to configure the settings/components, so for that reason along I won't be installing it any time soon.
Avast Antivirus Free should still be available (and should also get the Firewall component in due course), so I would say that is your best course of action.
If you also look at the Avast One for windows there is no mention of the Mail Shield either.
In the same way as there is no mention of the Behaviour Shield, but by all accounts this is because it is essentially integrated into the File Shield. So I just wonder if the same isn't true for the Mail Shield, possibly being incorporated into the Web Shield as your email essentially comes from the internet, just using a different protocol.
This may be an over simplification on my part. So I would say wait and see when Avast Antivirus Free gets a Firewall function, in the meantime you still have the Windows firewall.