A few things to consider on a site like this:
1) Choose a system that can easily integrate with a shopping cart AND which has a CMS. You should find decent integrations with Joomla, Wordpress and Drupal.
2) The best way to stay secure on these sites is to keep current with security patches and make sure you have a very strong password. These are two of your biggest threats for security as all of the above platforms have had security issues in the past.
3) You may want to spend the money on purchasing a shopping cart, but you may find some free options that will help you with the ecommerce side. The largest limitation that I've found with FREE shopping carts is that they sometimes have limited integration (i.e. Paypal only).
--If you think your volume is going to be decent, I'd recommend having your bank set you up with a web-based credit card solution. The fees are typically less than Paypal.
--I've used ClickCart Pro in the past (paid shopping cart) and was relatively pleased with it.