Single node availability group - is it possible? Is it possible to have an availability group with just one node in it? 

The reason I ask is because I am considering a distributed availability group spanning two single node always on availability groups thus getting away from the requirement for a WSFC. The two nodes being several miles apart which may mean a normal availability group may be problematic with data synching.
To specifically answer your question, yes you can create an availability group on a single node cluster.  I tested this with SQL Server 2017, and you can create an AG on a single node cluster using the "New Availability Group" context menu item in SSMS, or with the raw T-SQL.  However, you cannot create one using the "New Availability Group Wizard" item as it requires a second replica to be added to progress through the dialogs.

Based on your comments, I would expect that a two-node cluster with asynchronous replication and manual failover is a better solution.  There are no real benefits to using a distributed AG in this situation, and the benefit of a regular AG is that you can fail over and fail back as needed.  With a distributed AG, failover is possible, but failback is not.