I recently did my first shopping catalogue site using virtuemart and Youjoomla Youshopper template. There were many small problems, but that's to be expected in something so extensive as a shopping site. It would have been much more complicated in what you are calling "regular xhtml".
SEO looks fine.
Just don't underestimate how many small points need to be configured right in any package you choose for this. If light duty programming intimidates you, hire a pro. But if you're comfortable troubleshooting template logic, etc., you certainly can do it and the Youjoomla tools are helpful..