OsCommerce is a little poor in terms of functionality and in general its not so easy to customize (updating to newer versions is also a little too complicated imho).
Prestashop is quite flexible and offers many free extensions, not only paid ones.
OpenCart, as mentioned, and maybe ZenCart are not so bad options also, but I would personally go with Presta.
I would not recommend WordPress with a plug-in. Why would you use blogging software with an e-commerce add-on? Just use a program that was specifically written for e-commerce.
Although I would always prefer to use an application, specifically designed to serve an exact purpose, I would have to disagree with you here
Many people are tricked into thinking that many applications are set to serve an exact purpose, when its a more and more common occurance for those scripts to be improved and polished to increase their flexibility. WordPress is a great example of that - originally advertised as a blog software it has surpassed this target a long time ago. With its countless plugins and constant help from the open source community the script has opened possibilities, which cannot be bypassed. I have seen online shops, business websites, social media projects and many more, where you cannot even recognize the script on the first look.
Again, I do value and always recommend scripts, premade to serve the purpose of the customer as much as possible, but WordPress is my "soft spot" so I couldn't overlook this comment
█ Michael Vlastos
█ HostWired.com - Innovative Hosting Solutions since 2005 | cPanel, Softaculous
█ Personalized support | No site suspensions | No overloaded servers | Green Hosting
█ Multiple Backups | Average Response Time: 20 min | Toll Free: 1-855-ECO-HOST