On the question of 8.04 or 10.04, go for 8.04 if you are installing now. 10.04 is still in Beta. If you wait for 10.04, I recommend giving it a week or two after the official release date for paranoia's sake (in case some show stopping bug manages to get through testing).
There are a few good guides out there for hand-rolling a mail server using packages available in the standard repositories.
Another good option if the machine you intend to use is powerful enough is something like Zimbra. It is available free as the open source edition (the paid licenses get you a proper support contract, support is via official forums otherwise, and a few extra enhancements like fuller Outlook integration). Zimbra's full feature set may be overkill for you but it has the advantage of easier setup - install the base OS, download the Zimbra package, and follow the install notes - less detailed knowledge of the individual components is required. The browser-based client is very nice, there is an official full desktop client, it supports all the usual standards so you can connect your preferred mail program(s) to it, admin is not difficult, common mail server requirements like spam filtering are integrated, and they seem to be timely when releasing updates if security patches are needed. For the past three years I've run a Zimbra server for my employer (a small company, currently 10 active users) and one at home for my own use plus family&friends, and have found it to be very stable. There are similar alternatives too - I also looked into Scalix (amongst a few other options, including hand-rolling a simple mail server) when I was first looking at Zimbra. At the time Zimbra won out for my use but Scalix seemed to be a perfectly capable product too. At the time I looked at Scalix it was in some ways less featured (from our PoV) than Zimbra (though had one or two things Zimbra was missing) but also seemed lighter on system resource requirements - some time has passed since there may have been significant changes, so I suggest you have a look at the features sets and recent reviews for both to see which suits you better. There are other similar and smaller options too - try searching for things like
linux groupware if you want to find more to try. Zimbra and its ilk will need a more powerful machine than most hand-rolled solutions though still not massive by modern standards (at least a 1Gb of RAM, I've allocated 1.5 to my VMs and they cope with the load they see, and CPU of at least high-end P4 standard - so just about any new-bought machine should cope admirably for a small install).
What-ever you do (hand rolling with standard tools, something like Zimbra/Scalix or something more pre-fabricated but smaller then Zimbra/Scalix) I would recommend a dedicated machine (either a real machine or a VM - my Zimbra installs run in VMWare VMs) so you don't need to worry about an update to one server's OS upsetting another (so, for instance, you could install a particular web server on your file server without it conflicting with the one used by Zimbra).
So you have quite a number of free options available, some of them very powerful. Hand-rolling is not difficult if you have the time to learn the relevant tools (or already know some of them) and consider yourself a capable admin, though I'm glad I took the Zimbra route when I did.