I am complete newbie in Linux! Is a mater of fact I only have been twice in position to "see" Linux /presume Knoppix/!

I shall try to explain My position and any suggestion which provide solution is appreciated!

I have computer {MB Epox 8RDA3+ nForce 2 400Ultra,with AMD Barton 2500 and 512 Mb of DDR3500, HD ATA 80Gb} which need to be Mail server for 30-50 mail-accounts!
Those 30-50 accounts was on LAN with Win XP Sp2 computers with MS Outlook Express like a Mail-programs!
Mentioned computer /lets call him Server/ shall be connected on that LAN and true wireless AP /Mikrotik RouterBOARD 532/ to ISP where was domain!

My Questions:
1. Which distro /if that mean distribution of Linux like Fedora-SuSe..etc/ to use?
2. Which software to use for that Mail Server?
3. Where /web addresses/ to found "How to" or manuals or any kind of helpful articles about How to setup computer/OS for purpose which I describe as well as software which You suggest?
4. Does exist Live CD/DVD distribution which can be used for this purpose?
5. Is it posible to have Knoppix on USB Memory Stick or USB HD?

Like I said, any help was much appreciated!