eCommerce is incredibly complex in my opinion... and I have one

(opinion that is)
There are differences between shopping carts that organically built as a shopping cart and allow plugins either for free or third party purchase.
A CMS is generally built around different criteria and it's purpose is often for managing content that requires more than just a blog.
Wordpress in my opinion is a great blog, that's about it. I use it myself, as well as now installing my new magento /lightspeed server.
What you need to ask your clients is very specific questions about delivery - how are their products going to be delivered as that to me has been the one sticking point on different ecommerce solutions.
Shipping can be an interesting thing to calculate and take hours to get the algorithms or structure correct depending on product type.
Then there is the design element - if they have some custom design, then you need decide which store is able to be themed as close to your client's requirements.
You can buy an off the shelf design for magento for about 150.00 and it's pretty slick and your time is spent learning about how to put the store together rather than making it look fancy.
I am not as much of a fan of wordpress for shopping carts, as it's like me trying to make blog out of a shopping cart app - it's not quite designed to do that and if you are new to something and it's a learning curve for both, perhaps using a standard off the shelf shopping cart that you know works well - and there are free and well priced commercial ones to choose from, with some support mechanism to help you overcome your initial learning phase, then you should still remain friends at the end of the project.
Although I have no issues with wordpress - I think for a beginner to both, it has some quirks that might not make the wordpress experience as pleasant as it could be. If you are a wordpress guru- then that's another story.
Also, time constraints - ecommerce generally takes longer to put together than a standard website.
However, it's a very rewarding area - I love it. It's incredibly challenging but the outcomes are very nice.
In the olden days, back in 2001 > 2002 - I had this fabulous email marketing and ecommerce app which my guys wrote - it was so slick. People could buy a product, and then the vendor could transpose the purchasers details into the email app, and it would email them and upsell and cross sell.
What I learnt from that expensive exercise was... wait until the world is ready. *sigh* I really liked that idea... but I spent more time telling people what ecommerce was and what email marketing was and there were not enough people going online to sell. And then on April 16? 2000 I believe we had the .com gone crash V1 and people were scared. It's a different world now.
If I were you and new, I'd take up the offer of having your quote looked at - it could save you some headaches.
And I like cs-cart as well - it's quite a beautiful product.
That's my penny's worth.
Nina