"Is there a risk of a breach of security on OS X?"
If you have a computer and insert a disc; and / or connect a USB storage device or USB / Firewire external hard drive; or, connect to the internet - you are subject to risk.
However, no such actual breach of security of a Macintosh, running MacOS X, has been published.
"Are there any known viri for OS X?"]
Yes. In concept much code has been written; however, no such code has been circuiated. None what-so-ever.
"Is there a possibility of key logging?'
Yes, there is always the possiblity - of anything.
Can such software be installed without your knowledge, say from a received e-Mail message? No. Especially, if you create (a) user account(s) for surfing the internet for information, file transferring, and e-Mail messaging (and not using the adminstrator's account - unless when needed to install software, etc.).
By the way, there is a commercially available '
key-logger' application for the Macintosh.
Also, writting a Macintosh key-logger (or background screen capture) application is not too difficult. The difficulty is in installing it without the user's knowledge.
"We used Mail on our old imac and everything was fine, but I need your opinion so I can persuade him that it is worth using mail."
I am a fan of System 9.2.2 and earlier Macintosh Systems. I still use such for productivity, a truly User Friendly interface, and such (compared to MacOS X, 10.0.0 - 10.4.2); but, did you know that (all) the actual 'Mac' viruses (released and which infected, some destructively, many Macintoshes), were written for pre-Mac OS X Systems? More like System 7.x and earlier.
Here is a list of pre-MacOS X Macintosh viruses (obtain from within 'Disinfectant' v.3.7.1):
ANTI, CDEF, CODE 1, CODE 252, Frankie, INIT 17, INIT 1984, INIT 29, INIT 9403, INIT-M, MacMag, MBDF, MDEF, nVIR, Scores, T4, WDEF, and ZUC
Naturally, the list may not be complete; nor, does it include any potentially damaging 'Word' micro viruses. Again though, I know of no publications indicating the Mac platform with such a compromise.
If you and your partner are fine with the security of System 9.2.2 and earlier, you should feel equally or more secure with MacOS X.
As far as, 'I would like to set up the mail so that I don't have to log into our numerous webmail accounts.', if the webmail accounts can be accessed by a stand alone e-Mail'er application ... you have many choices with either MacOS X or System 9.2.2 and earlier'.
There is 'Mail' - which comes with MacOS X, '
Eudora' (shareware), and '
Thunderbird' (freeware, MacOS X only) - to name but a few. See
this web page and
this web page for additional Macintosh e-Mail applications.
A final note:
Anti-virus software for the Mac, that is a Mac running MacOS X, does not search for and remove Macintosh viruses. It searches and removes Windows viruses and Word macros (which could potentially do a Mac damage) - that you have have installed, downloaded, or received via e-Mail. I have yet to see one Macintosh anti-virus software putblisher provide a list of known Macintosh specific viruses (System or MacOS X), let alone known Mac damaging 'Word' macros, not one.
