Paul, while I'm not familiar with your specific requirements, usually using WordPress (blog system) as an E-Commerce platform isn't a good idea, while there are plugins that will help you set it up, you'll end up with a Frankenstein monster that will not scale nor allow you to freely upgrade/update, in short: your TCO will raise every month (and well, one of the known flaws of WordPress is security...)
I've developed WordPress sites for big non-profits like GirlScouts of America, fortune 100 companies like Condé Nast, and many others in the last years. What I can offer is: take a look at what you're trying to build and recommend the best approach (for free). Feel free to email me at firstname.lastname@example.org.
As for recommendations, I would go with AD:60 (http://www.ad60.com in NYC) They are good, expensive, but good. Another alternative is Luminopolis, cheaper and good as well (http://luminopolis.com/, they're based in Kansas)
Have a nice day,