I love my X30 from Dell. Not their newest Pocket PC (so their newer ones may have a better spec) but does everything you want except it doesn't have a USB port for your pen drive but it does take an SD card for storing your research on.
I have the fold-able keyboard with it that plus into the bottom for when I need to sit and do some serious typing but I find the stylus o.k. for short notes or emails.
It has Wifi and Bluetooth, runs Adobe Acrobat very well and handles even large PDF files (600+ pages) with relative ease.
My only gripe is that Windows Mobile 2003 sometimes crashes it but that's a software issue rather than one with the hardware.
I really like the landscape mode too which allows you to read files and web pages easier.
It comes with pocket Word, Excel, and Outlook so you can use it to update your appointments and emails while out of the office.
Seems to tick most of the boxes that you have - my only reservations on recommending it is Dells ****ty customer service.
