This request is pretty old, but in case Bear or anyone else is still interested, let me offer an alternative solution. I've got a program available that is a fairly comprehensive download manager (doesn't accomplish item #3, but it could be done with some modification). It is written in PHP and MySQL, so it wouldn't tie directly into Links SQL as a plugin. However, one of the plans is to make the User table flexible enough to tie into other programs, so it may become possible to incorporate it into your existing database.

Rather than advertise for it here, contact me at the email address in my profile if you are interested in more info.

Dan