> is Postfix only for outgoing mail (SMTP) ?
No, Postfix will listen on port 25 of your server, and receive incoming mail, too.
> Can i setup Courier as IMAP server (IMAP/POP3) and Postfix as SMTP server ? Is it a stupid soulution ?
No, as far as I know it is quite a good and common solution (though I do not use them myself).