[Exim] exim-newbie needs your advice

hi

i'm using exim on a small network consisting of two debian systems (eg. A
and B). A is connected to the internet via a adsl-modem, therefore being the
gateway for B.

now: what configuration of exim would you recommend?

what i tried:
on A i use exim as "Internet Site using Smarthost (my isp's mailserver)".
B, i tried to configure so it uses A as smarthost.
this way i can send mails to the internet from both computers, send locally
on A and B, also from B to A, but NOT from A to B, because A always tries to
send the mail to it's smarthost, which is the isp's mailserver.
anyone still with me?

am i using a totally wrong approach? should i configure B as a satellite
system? don't i need exim at all on B?

normally on a lan, is exim running on every machine, or only on the
"mail-computer"?

thank for helping an unknowing who is learning from scratch.
mario