Well, that Zaurus was pretty spiffy, and at 150 euros would've been a pretty nice deal, but alas, no dice.
Perhaps that Zipit thing is worth the 100 bucks, but it would be pretty much purpose-built for this use.
I still think an iPaq is a decent device for this sort of thing, but I think they're a bit too expensive for what you're getting.
I'm hoping Metatron will pitch in with some suggestions here as he seems to be the more knowledgable one on this topic.
I got my Zaurus from a diffent site, so it wasn't me! I got mine for 100 euro's, that's about 130 US$ I suppose. I won a Zipit offa ebay for $60, so I'm gonna experiment with both soon.
But on the question of which PDA to reccomend: that Zaurus thing seems to be the best choice, since you get a color screen, fairly modern PDA with all the functionality you'd expect, plus you can just run Linux on it like that. There's lots of security tools ported for it already so it can be turned into a packet sniffing, warwalking little box of fun in a short time.
But the Zipit is definately your cheapest choice, and requires some cool hacking to pimp it with extra memory and stuff, and comes standard with good Wifi reception.