Have a look at the Archos devices. You can pickup some of the older models with Wifi as refurbished.
My Archos 5 is my, surf, watch movies, store data, listen to music, and play games device. The current 5 and 7 are called "Internet Media Tablets".
With Opera as the browser and flash support its really a nice internet experience for a small device.

If you haven't looked at the F5 product line you should. The ability to use TCL language to write "iRules" and shear performance of even the smallest device is amazing. The devcentral.f5.com site is also great and allows you to gain from others experience. With an F5 in front the rest of the systems behind can be simple and cookie cutter with no complex setup. The F5 will handle persistence, load-balancing, and once you have your setup you can forget them for the most part.

For the FTP server part, you just need some Linux boxes running your favorite daemon and a shared storage for the files.

--russ