I would go with the Merchant Metrix Cart. (still on their old site:
Shopping Cart Software SEO Rated #1)
Without trying to assume anything bad, it seems like building and maintaining a website shouldn't be the thing where your client or you should be spending most of your time at. Better to use a system that does all that work for you. When you want to add a page, update a page, update a product, add a product, update a category, create a category, add a product to more categories (without creating duplicate content on different URL's), create options for that product, create forms, create landing pages for the PPC purposes, etc. etc. etc., you do not want to spend most of your time on the technical aspects, but more on the marketing aspects.
So if you don't want to waste too much time on the technical aspects of building and maintaining the website it self, I would definitely go for Merchant Metrix. We´re using it on some client sites as well and its a blessing. Easy to learn how to use it. Only thing that requires HTML knowledge is to set up the templates (which they can do for you), after that it is smooth ride.
Ah, and best of it all is (from an
SEO point of view) that it is all optimized. You can create custom titles and meta tags for each page, category page and product page. When you visit a site running on Merchant Metrix you'd think they're all hand made pages, even the whole checkout process uses static URL's. Not a single page that uses parameters in the URL's. Also they use ajax tecnology to prevent unnecessary page reloads. (great for monitoring visitor statistics).
The system automatically creates shopping feeds for sites like froogle and bizrate. Also you can add PPC tracking codes without messing in templates, it's all prepared for that. If you're building the site in an existing domain, you can setup 301 redirects without knowing how to technically do that. Just give the 2 URL's and the system does the rest for you. Discounts are easily added, from bulk discounts to discount coupons.
Ah I almost forgot to mention the automatically created XML sitemaps for Google Webmaster Tools. Automated shipping calculation I almost forgot as well,... actually there´s a lot more,..
It's really a complete system and you'll like the price too I think. Goes per month and not expensive for a small site like you are planning.