My first suggestion is that you format the PDFs to the screen size of whatever reader you end up getting. There are several devices that can handle PDFs, but they do a better job when the PDF doesn't have to be resized.
My second suggestion is that you hire a couple people (who already own a similar ebook reader) to run tech support just for the reader during the conference. You should probably hire them a week before the conference. If the 3k devices each require 10 minutes to set up then you are looking at 500 man hours of work.
I have a device to suggest.Your Wifi requirement is a killer. Very few readers have Wifi. If not for the Wifi, I would recommend the Hanlin V3. It has the broadest format support of any reader, and the manufacturer has been releasing regular updates. It even supports Powerpoint.
Here is the product page, and here is a
PDF spec sheet.
I also have an outside the box suggestion. If you want broad format support, and you want Wifi, then you might want to look at the Asus eee 701 running WinXP. The upside is that there is almost no format it can't read and it retails for $400. The downside is that it has a laptops battery life. (It is a laptop after all.)