>> An old Powerbook 180 from 1994 was offered to me, and I'm quite
>> interested in it (I never had any Apple HW).
>
>     http://www.lowendmac.com/pb/180.shtml
>     http://docs.info.apple.com/article.html?artnum=112192
>or
>     http://www.lowendmac.com/pb/180c.shtml
>     http://docs.info.apple.com/article.html?artnum=112212
[snip]
>> Another HW question: what about the HDD? It's SCSI, but which size?
>> 2.5in?
>
>Someone else will have to answer that one.

  yes (SCSI) and yes (2.5").


  if you can't find another 2.5" SCSI HD, you may be lucky to find
an "Apple SCSI System Cable" M2538LL/A so to be able to connect an 
external SCSI HD of your favorite size
  look at the pictures halfway down at
<http://www.byc.ch/scsi/scsiterm.html>
for proper termination...

  you may also read
	<http://docs.info.apple.com/article.html?artnum=14133>

  I wasn't able to find a picture of the cable  :-(

Stephan