eCommerce Site Help

I'm sure this is posted a lot, and I looked threw the topics here and on other websites but I didn't really get the answer I was looking for.

But, I am helping to design an ecommerce website.

Basically, there is an already existing store that sells bridal gifts, grad gifts, baby shower gifts and such. We want to expand the store online.

Some requirement:
- good SEO is a must
- Needs to accept several payment methods
- easy to update
- pass validation
- very good security
- not much maintenance once site is complete
- something that wouldn't have that much of a startup cost
- not much hassle on set up.
- easy to use with a payment gateway.

I've looked at, and tried out Shopify, Joomla Virtuemart, Drupal Ecommerce, and osCommerce.
I've read very mixed reviews on all of them.
I've also read about cubecart, and http://www.bigcartel.com/
To me it was an overall consensus that oscommerce is awful, and Joomla + drupal both have their faults.
What i like about Shopify, is that you dont have to pay them anything if you don't sell.

I'm really worried about security, and I don't want anything to go wrong.
If anyone has tried out any of these, a review would be very helpful.
Any help at all will be greatly appreciated.
Thanks!