Aman,
There's no direct need for UI5 for your purposes. This can all be done with the WebDynPro UI. However if you want to expand on what is offered in the standard UI, then yes, UI5/RESTful will be very helpful.
As far as IDM or GRC, that's a loaded question to which there is no "right" answer. All I can tell you is that based on my experience as an IDM consultant, the best thing you can do is document the needs of your organization and then map that against features and functionality of the two products. That should be the ONLY indicator of which direction to go.
<Shameless plug>I'll be talking about this and similar topics at this year's TechEd/dcode in Las Vegas this October</Shameless plug>
