Wordpress.

It's the cheapest, most capable, most user-friendly CMS available. It has the largest community, which means it has the best support and greatest plugin availability.

There's nothing wrong with running E-Commerce on Wordpress also, as long as your payment solution does not include storing sensitive data such as credit card numbers in the database.

"How do I improve security" is a question too in-depth for a forum comment. You need to spend some time researching that using your favorite web search, in my humble opinion. Just keep in mind there is not one cure-all solution.