Ahh, to be a college student again. I'm glad I didn't know what PDAs were when I was in college, or I'd be in the same predicament as you. I typed up my college reports on an old Commodore 64, how's that for scrimping?
Anyway, I can't think of a PDA that is new with the features you want for the price you want. Dell has several Axims that are high quality and have CF slots, and would fit in your price range. They aren't upgradeable to WM5.0 though. Here's a link:
http://72.14.207.104/search?q=cache:...s&ct=clnk&cd=1
The Ipaq HX4700 is supposedly upgradeable to WM5.0, and you *may* be able to find one in your price range. Bottomline, if WM5.0 is most important, you're gonna have to wait till prices come down.
I would also recommend looking at devices with a keyboard, as they are better at data entry--my fav is the iPaq 4350 series. No CF and has WM2003, but it's the most stable PDA I've ever owned, and it remains my workhorse for about 2-3 years now.
Let us know what you get.
Edit: the link I gave you above is for an Axim 30 which does not have a CF slot. Most of the others do, including the 50 series which does have a slot, and is upgradeable to wm5.0 (but may be out of your price range). Check around on the various Axims, I have a feeling they will meet your needs.