Hi,
As Bijay explained a purchasing document is not configured at plant or company code level.
You need to understand what kind of data is existing at which level in the SAP system.
Purchasing document types are at the highest level and should represent the type of process that is used.
Same thing goes for the purchasing group so you are not able to restrict the selection options on these settings and the only thing that you can do is restrict access via authorization settings.
If a user selects the wrong document type and the system returns an error on saving the user can change the document type at that moment. It will not delete all the data, some data could be removed or needs to be changed based on the field selections that are set for each document type.
Only other solution is to create transaction variants to restrict the selections via these transaction variants.
Regards,
Peter