Hi Saket,
If your approval is going to happen via a custom workflow then you can develop an OData service which first fetches the work items which can be displayed in the UI5 app and then handle approve/reject/update depending on your requirement on a non-7.4 gateway system. Ultimately the workflow will run on your ERP back end system (which can also be lesser than NW 7.4). On the gateway front end system you won't need any specific components other than the Fiori infrastructure components.
Regards,
Saurabh