the biggest difficulty is that paypal standard is not a real time payment gateway.
with digital goods, the customer will expect to be able to download as sone as the transaction has gone through, but with paypal standard, that is not the case.
with paypal standard, the user goes to the paypal site, they choose the payment method, if the payment method is accepted, they are directed back to your site.
at this time, the transaction has been placed in a queue, the transaction will be processed on a first come first served basis.
The customer cant download anything yet, because the status of the order is unknown, it has not been processed with the merchant bank yet, and no funds have been transferred to your account.
see this post for a description of the process: