Hi Rajiv,
You shouldn't develop any business content inside Liferay since portlets only care the view side of your model.
Instead, you shall expose your application with frontend services (You can use Apache CXF for example). Then you can develop all the exiting portlets you can imagine in terms of ergonomy, style, functionalities. Your portlet will have access to your application through the services you developped.
The last thing to care about is authentication. The best way to deal with it is to integrate your stuff with some SSO server. Liferay already handles plenty of SSO server connectors which will make your work easier. You just need to implement it inside your app. OpenAM for example is very quick to integrate since it only relies on two Web Services. But you can use CAS as well or the one that fits your needs best. Just pass the token along your Web Service request and let your application validate it upon the SSO server to retrieve the connected user.
