SharePoint 2016 – Upload, Overwrite, download and Delete Files in Document Library through WCF

Background: Our company has Oracle ERP as the enterprise HR & Finance solution. For document management(DMS) SharePoint 2016 has been proposed.

Catch: In current implementation ERP is calling java Web services for performing required operations. And the ERP team wants least changes in their client application.

Requirement: ERP application needs ability to perform below operations on a SP Doc Lib;

1. Upload File
2. Overwrite
3. File Download
4. File Delete File

Options for Implementation;

1. REST API – The OOTB SharePoint REST API is ideal option we seem however, currently the ERP application is call a Java Web service to perform these operations. And there is very little possibility they redevelop their client application to work with REST API (more over call REST URLs through JavaScript).
2. WCF Service – WCF (BasicHTTP) service seems another option wherein similar to java service function there will be c# functions and client application with minimum code change and config changes can continue to perform document operations.
3. Web Service – Plain Old asmx services

Kindly suggest what is the most ideal approach to this requirement. Links to samples are highly appreciated.

Thank you