Big time loaded question.
99% of the people here are going to point you towards freeware for the mail and webhosting and in my opinion that's not the way to go.
Hardwarewise. Build a cheap dual CPU Intel system. (Abit VP-6 and twin 1 gig's, even 800EB's.) AMD if you prefer, but I would rather see it be Intel because the solidity is there. Go with a ton of memory. Put at much memory as you can possibly afford. I don't care if you use SDRAM, DDR, or Rambust, just max it out. You will be happier, and so will your system. Too keep the price to a minumum, look for something with IDE RAID. Get 4 identical drives and set one pair in RAID0, the other pair in RAID-0 and mirror them. But your email system and the webpage on those drives.
Think about some kind of removeable media for backups. And no, a CDRW isn't the way to go.
Softwarewise...simple. Microsoft. Get Exchange server 2000 and Windows 2000 Server at the minumum. Sure you can go Linux, keep in mind that Linux had MORE security flaws than Microsoft did in 2001, and the way things are going it's going to be the same for 2002.
There is one thing to keep in mind. You can do it cheap if you want, and it may work for your LAN box, or your normal desktop machine. However, when it comes to servers, you WILL get what you pay for.
If you are going to go Microsoft for the email take the time and learn exchange before you throw up an email server. If you don't you will be open to relaying other mail and it will only be a matter of hours before you are blacklisted for spam that you dont even know is running through your system. Trust me on this. Also take the time to learn how to secure a Windows server system. There are great tools on Microsofts site like urlscan.exe. That program alone is needed to stop nimda.
You're going to need an antivirus capable of scanning both the email and the local machine. Norton Anti-Virus Corporate Edition is the way to go. As much as I hate Nortons I run it on my domain server at home. It's one of the few, if not the only one that can scan both.