Take a look at these pages Microsoft Dynamics CRM SDK
] - A web service, as in Web Service Walkthroughs
] is probably your best choice - think in terms of operations, and only expose what is strictly required.
"Moreover,will there be any need of load balancing (web farm ) for 6000 to 7000 total number of external users (ASP.NET 4.0 portal users)"
Depends on the workload - not the total number of users, so your milage will definitely vary ...