Disclaimer: I have never worked with and I know nothing about the Orion API.
Currently adding nodes to Orion takes a significant amount of time for us. How much of the process of adding and configuring nodes in Orion can be done via the API?
When you download and install Orion SDK, the example how to add Node via PowerShell is here:
c:\Program Files (x86)\SolarWinds\Orion SDK\Samples\PowerShell\CRUD.AddNode.ps1
The sample is also available here: OrionSDK/CRUD.AddNode.ps1 at master · solarwinds/OrionSDK · GitHub
Will this also allow me to configure custom properties, select which items I want to monitor on the node and assign application templates?
Custom properties - yes
Select which items to monitor - limited flexibility here. This shows how to selectively add interfaces OrionSDK/NPM.DiscoverAndAddInterfacesOnNode.ps1 at master · solarwinds/OrionSDK · GitHub, but there's no granular control for other areas.
Assign application templates - yes. See OrionSDK/SAM.Application.ps1 at master · solarwinds/OrionSDK · GitHub for examples. Note that there is currently no API for customizing application templates, just assigning existing ones to nodes.