As someone who had to put up with dial up for too many years, I can only state automatic updates are not necessarily deal killers. As long as your surfing habits don't rely on constantly downloading large files, most people will not even notice automatic updating. Even on dial up. And there is a reason for it, your computer CPU will time share
when you are downloading many files at once, so it not like your computer will be paralyzed while downloading security updates. And right now, as I read this thread and compose an answer, my computer is basically doing nothing which gives even dial up a chance to catch up.
As for me, the AV I now recommend is the avast free one. I used to recommend Avira until its auto updates became rather flaky.
But still, I feel only fools rely on just an AV for a security system, which gives you even more security programs to keep updated.
The last thing to say, is its super important to find the right dial up modem. Even new expensive dial up modems can be very slow, and the best preforming dial up modems I have ever found tend to be super cheap no longer made agere win modems with the scorpio chip sets. With those I could get 40-45 kbits a second down, while most other modems delivered 10-15 kbits a second down.
The last last thing to say as an edit in, even if you your only internet option where you live is dial up, there are still some survival techniques you can use. Because public libraries often have broadband options, so you or a coworker can download needed large downloads, burn them to a cd, or put them on a usb pen drive, and freely share them. And if you have a lap top, one can often find WIFI hotspots and use that to download large files in a tiny fraction of the time it would take on dial up. When it could often take an hour on dial up to down load a 20mb file.