In order to integrate PayPal with my ASP.NET website using C#, I was told that "Sign up for a PayPal developer account and obtain API credentials. I will need to create a Sandbox account to test my application."
At present, I hope to use Live Mode of Paypal instead of Sandbox Mode, must I upgrade Paypal account to business account ?
To process live payments, you need a live business account. There is no reason not to upgrade, having one is free.