1) Do all the machines need to run Jaguar?
No, only the mail hub. The other machines may use
any OS. The only caveat is that the OS support a compatible eMail client.
Note: From your original description, i thought that maybe you
needed the capabilities of IMAP, but from your second post, it sounds like an ordinary POP or POP3 system might do too. Many mail packages support both, but POP/POP3 is still more common.
2) Will I need any additional hardware?
No. If all your machines can talk to each other now, that is all you need.
Man, Fetchmail doesn't look to be for the faint of heart. I've got about 49 minutes of experience using OSX.
Ha ha. Well, i know this UNIX command line stuff can be intimidating at first, and some people do seem to have problems getting the configuration file right, but i found it to be fairly painless and straightforward. But it's been a couple years since i delved into Fetchmail, so perhaps time has softened my recollection? The good news is that Fetchmail is installed as part of Jaguar (assuming the BSD subsystem was installed, of course), so you don't have to hassle with downloading and installing it. You need only worry about configuring and invoking it.
Anything commercial available?
There may very well be, but i can't recall any offhand. Fetchmail is what most people use.
Now it is important to point out that Fetchmail's job is simply to pull eMail into your mail hub. Since your desktop is "almost always on," you could omit Fetchmail if you set up your machine to accept eMail as it arrives, but that's a whole other can of worms, and potentially more complicated. However there definitely are commercial software packages to solve that problem. It also requires a static IP.
The reason why this approach would work is that, typically, when eMail can't be delivered immediately (i.e. because the recieving server is down), the sender will retry sending the message approximately every hour for five days.
But Fetchmail solves a number of problems (such as eliminating the need for a static IP), and minimizes the chances of lost mail (as mail is delivered to an always up mailserver, and then downloaded to your part-time server), so it might be to your advantage to at least attempt Fetchmail. But i can understand where you might not want to get into UNIX command line stuff (i.e. using the Terminal.app).
One tip, however, which may help a lot: Many tutorials suggest using command line editors for editing configuration files. I have never understood this at all. This is akin, in my mind, to telling drivers to buy crude oil and refine their own gasoline. A file is a file, and it does not matter what kind of editor you use to create or edit it. I never use command line editors, even though i do a lot of command line work.
All you have to watch out for (when using GUI editors) are two things: 1) line endings must be UNIX style endings, and not traditional MacOS style, and 2) files need to be saved as plaintext, and not Rich Text or the like (i.e. no formatting). MacOS X's TextEdit can do the job, but many people prefer BBEdit Lite. Personally, i like Tex-Edit Plus. But any of these can do the job, so it's just personal preference.