SQL Server: Replace invalid XML characters from a VARCHAR(MAX) field

Question

I have a VARCHAR(MAX) field which is being interfaced to an external system in XML format. The following errors were thrown by the interface:

Answer 1

It is safe to use VARCHAR(MAX) as my data column is a VARCHAR(MAX) field. Also, there will be an overhead of converting VARCHAR(MAX) to NVARCHAR(MAX) if I pass a VARCHAR(MAX) field to the SQL function which accepts the NVARCHAR(MAX) param.

Thank you very much @RhysJones, @Damien_The_Unbeliever for your comments.