I am still in the early testing stages, but I'm planning to use PayPal standard. And with that, I think I've answered my own question -- that UE doesn't work with PayPal Standard because the emails are sent by PayPal.
...but that raises a separate issue. If I'm trying to sell digital downloads, I still need to be able to send some sort of a confirmation email with a link to the download page. I am under the impression that that's ordinarily accomplished in the receipt that's sent to the user after a purchase. If I'm using PayPal Standard, how do I accomplish that?