Such processes on a web site -- and many more -- are created by
dynamically combining formatting and function code with database
contents. Usually this is all put together using PHP code and a mySQL
(or similar) database.
You had the right idea when you looked at Magento. I would use Zen
Cart (
zen-cart.com) except that I know that Zen Cart does not come
with that particular ability built-in. I'm not sure Magento does
either.
These e-comm packages are not designed to be added onto an existing
site. But you can use their built-in methods to recreate your site
within them and can style them to look like anything, pretty much.
There is a quite steep learning curve associated with this sort of
software, and the templating process is not the least of it. Your best
bet would be to join the forums for any such software you wish to
consider and to read up on them as much as you can. Ask questions on
the forums -- in particular about that comparison requirement -- to
find how others have done what you want to do. Check out the free (and
paid) templates available, which you can use as a starting point for
your own.
Rob
PS
You can see my two e-comm sites, both built on Zen Cart, by going to
thistledance.com and clicking either the wholesale or the retail links.