I've not used osCommerce but have studied it in some detail.
It is a free open source eommenrce system that is suited to most php / mySQl providers. It is certainly configurable and the admin interface looks professional out of the box. You'd want to configure the front end to meet your ecommerce site(s) needs.
Port 25 is almost always the SMTP server used for sendin emails. Almost all hosting compaies will provide you with access to a SMTP server to allow you to send emails from your scripts. this will usually be something like smtp.mydomainname.co.uk or smtp.hostingcompany.co.uk. you get the idea.