NAV 2017 Web Server install error

Hi folks - I'm having no luck getting the NAV 2017 web server installed on a Windows Server 2012 r2 box. The NAV 2017 service tier, and SQL 2014 are running on the same server (very small customer). The sequence is install, error, rollback. This is a system that had the NAV 2013r2 web server running before upgrade.
I have removed and re-installed IIS in the process of trying to fix the issue. After rooting around in the event log I came up with a WIX log that complains about not finding Powershell 3.0 - The server has powershell 4.0 and 2.0 installed via windows features, with no option for powershell 3.0.

I found a wix log file related to the installation that contains the following line (log file attached) ...

Property(S): NAV_POWERSHELL_MESSAGE = Microsoft Dynamics NAV Setup cannot complete because an installation prerequisite is missing: Windows PowerShell 3.0. You must install the relevant prerequisite before you continue with Microsoft Dynamics NAV Setup. For more information, see http://go.microsoft.com/fwlink/?LinkId=317819.

In the windows application log - I'm seeing a message that says ...

Type: Microsoft.Dynamics.Nav.Management.Exceptions.NavCommandException
SuppressMessage: False
SuppressExceptionCreatedEvent: False
FatalityScope: None
Message: <ii>A web server instance already exists.</ii>


I am not sure if they are related, but one happened at the same time as the other...
Any suggestions would be appreciated!