Hello,

> I've been offered an iBook G3 at 600Mhz.  (I don't have more information
> about it yet, but I think I can expect it to have, at least, 128MB of
> RAM, from what I've seen in some pages.)
There are 3 different iBook models with 600MHz G3s, all have 128MB RAM soldered on the mainboard.
Two come with a 750cx ( 256kB L2 cache ) and a Mobility Rage 128 with 8MB VRAM one has a 750fx ( 512kB L2 cache ) and a Mobility Radeon with 16MB VRAM.

> If I get it, I wouldn't like to have to buy any software to make it
> usable.  Furthermore, I doubt Mac OS X could work fine in it (hey, I may
> be wrong!).
At least the version with 16MB VRAM should run OSX fine when you give it more RAM - at least 256MB, preferrably more.
The others aren't really slow either but some fancy acceleration bits in OSX work only with Radeons or similar newer chips, not with mere Rage 128, and software emulation will slow things down a bit. I have no idea how big the impact would be or if it's noticeable at all.

>  So my idea would be to install NetBSD 3.0_BETA on it all alone.
/should/ just work...

> Based on my previous experience with the mac68k, which needs a copy of
> Mac OS to boot NetBSD... is this required in NetBSD/macppc?  I think the
> answer is not, but I'd like to be sure.
No. NetBSD/macppc boots via OpenFirmware.

> Secondly, how well is the hardware supported by the NetBSD port?
> Networking? The airport card (dunno if it has one)? Graphics?
The graphics chips are supported by XFree86 but may need additional hacks ( my iBook G4 does )
Audio should work, onboard ethernet should work too, the AirPort card is supported as far as I know ( the Extreme isn't, but these machines can only take the non-extreme version )

> PS: And The-non-NetBSD-related-question: does 220 euros sound like too
> much for such a computer?  I did a bit of research and similar
> second-hand iBooks seem to be more expensive.  But dunno if it is worth
> it (no idea about ppc prices...).
Depends which model it is and what else you get ( RAM, AirPort card and so on ) - if it's the one with a 750fx it's definitely a good deal. 

You may find this interesting: http://www.everymac.com/systems/apple/ibook/index.html

have fun
Michael