The ipaq 214 or 114 would suit what you need.
Buy HP iPAQ 114 Buy HP iPAQ 214
They are a similar spec but you would probably see more benefit in the 214 if you're doing a lot of documents or spreadsheets due to the bigger screen.
The only thing these wont do is connect to your flash drive as PDA's don't have a USB host. What you can do instead is get a large memory card for the PDA and store your documents there instead of the flash drive.
We sell an 8GB card at the moment.
Buy Kingston 8GB microSDHC Memory Card with SD Adaptor
The PDA's come with a USB cable to connect to a PC to transfer data so you can use the PDA instead of the Flash drive.
These have Pocket Word, Excel, Powerpoint and Outlook. You can open and edit documents and spreadsheets and also view presentations. Excel will not run or save macros just standard charts and sheets.
They will connect to your phone using bluetooth so you can access the web and they also have wifi so you can access wifi hotspots for internet access. Antivirus software is available for PDA's although its not something we sell at the moment.
We stock case's, car chargers, screen protectors and folding keyboards if you need accessories.
Let me know if you have any questions.