I do not recomend using paypal standard for setting up a download center, the big problem with paypal standard is that it is not a real time payment gateway system.
I would strongly recommend using PayPal express or paypal payments pro instead, the difficulties you are going to run into setting up paypal standard for this are just to great.
with paypal payments standard, the transaction is not completed in real time. Paypal uses a system called IPN to send a post to your site when the transaction is actually processed.
you search this forum for many threads where i talk about the difficulties of using paypal standard and IPN.
