SAP Gateway exposes the functionality as an OData Service.
Java Web Dynpro does not allow consumption of OData services. It only supports SOAP based Web Services.
No need to JCo while using OData services.
Preferred UI would be SAP UI5. Its a niche market and requires skills on Java Script. However, its not fully tested on many customer production environments. Its a risk but worth it as SAP is projecting UI5 as their preferred UI in their UI roadmap.
Check the compatibility/feasibility of deploying UI5 applications on existing 7.3 portal server.
Best Regards,
Sharath