You can disable the thank-you email by checking Enable Templated Email in the product's settings but leaving the Email Message field above that blank (our system can't follow a template to generate the email if no template is provided); however, this means the buyer would be unable to claim their download if PayPal had not completed processing the buyer's payment by the time that buyer finishes checkout and proceeds to the thank-you page, and this is why we normally send a thank-you email with a link to the download page once their payment is actually completed. For technical reasons it is not practical to include a direct-download link in the email itself, so we haven't programmed any way to do that.
To bypass this issue, you can go to Seller Admin > Payment Preferences and UNcheck "Wait for pending payments...", so buyers who finish checkout could always proceed to their actual thank-you page with the download link; however, this means you would assume all risk that a buyer's payment could eventually fail after they've already finished checkout and claimed their download -- e.g., eCheck-funded payments can take several days to complete, and even bank transfers may be not-quite-instant depending on how their bank handles them. It would also still be possible for the buyer to bookmark their thank-you page to return to later, e.g. in case they need to retry their download due to some glitch or mistake that stalled or aborted their original download attempt.
Our own, built-in affiliate system would track referral commissions accurately through our order-handling backend without depending on thank-you page visits, which would also prevent duplicates of the same conversion from being tracked. We do not support multi-level marketing types of tiered affiliates nor have any plans to add that, for several reasons. Considering the massive undertaking that building such a system would entail, there really hasn't been much demand for it, so that would divert a lot of effort from other long-awaited, popular-demand improvements, making very little added appeal as a return on that investment. There's also the high potential for fraud and scams on both the merchant and affiliate sides that multi-level systems would tend to attract, and we simply don't wish to draw that crowd and attendant support headaches to our system (we've already had plenty enough problems due to just one merchant who tried to use our current affiliate system to conduct an upfront-fee scam). Finally, it's really just too complicated a feature for the simple, basic, easy service we are deliberately designed to provide, and many of our clients already have enough difficulty understanding our simple, single-level affiliate system as it is.