> What is the free space needed to install all the distribution + X_Windows 
> + Kernel_sources .

The following is the list of relevant partitions that I have:

Filesystem  1K-blocks     Used    Avail Capacity  Mounted on
/dev/sd0a      107407    33275    63391    34%    /
/dev/sd0e      107407    52597    49439    52%    /usr
/dev/sd0f      375967   209288   147880    59%    /usr/src
/dev/sd2g      300702   180721   104945    63%    /usr/X11R6

In /usr/X11R6, I have both the binaries and the full source.  You get
the idea how much it takes by adding up the "Used" field of each line.
Only for the things you've asked, 500M should suffice.  Of course this
does not count the home directories and all the local junks I've
installed over the time.

> I know it is possible to make PPP conections . But I haven't a static IP 
> (eg : DialUp IP)

It doesn't matter.  PPP as a standard protocol includes the ability to
negotiate the IP addresses at the both ends.

ken