Hello,

I have paypal working on my site.
I don't have API credentials so it works without. Just make sure you use the correct emailaddress for receiving payments.

IPN works automatically. I don't think you can turn it off.
I haven't found Paypal Data Transfer anywhere.

Make sure the submissionmethod is: Submit the whole order as a single line item.
Paypal server should be 'live'.

It works almost right out-of-the-box.

Good luck,
Sanne