1. What is a reasonable price for a second hand device?
2. Is it worth the buy one, because it a rather "old" device?
I'd buy another Zaurus in a second, but that's just me. There's quite an active community, and I don't see them going away any time soon.
3. Is the OS still up to date, or should I try an other OS if possible?
The OS is probably at least semi-out-of-date. Cacko would be a better option (I think it exists for the 6000, but I'm not 100% sure). If you want a "desktop-like" OS, look at PdaXrom. The version for 6000 is kind of old, but it works, and there are lots of packages for it. I would recommend staying away from Angstrom as a newbie unless you really
like reading docs, as you will most likely get flamed at for asking about anything that is covered in the docs. I also find it less customizable than pdaX, but YMMV. That aside, it is up-to-date and probably the most developed distro for the 6000.*
4. Can you give me some advice, if it is worth to buy, wich programs I have to look for.
Programs vary with which OS you choose, for "worth it to buy," see above and below.
5. Is it possible to sync with Ubuntu Linux? Because I read it was Windows Software only.
I think it is possible in some cases. With pdaXrom and the KDE PIM, you can connect it via USB and copy the databases over. I think there may be a better way, but I never use/used my Z for PIM, so I don't have much idea as to what can be done.
Now for a couple of questions:
Why a 6000 and not, say, a 1000 or 3x00?
What do you want to use it for?
Hope that helps.
*I, as a rule, love pdaXrom and dislike the other ROMs, so my opinion/advice is likely to be skewed.