First of all, you are telling us that OS Commerce is limited, but you are not pointing out exactly what the limitations are. If you paid, 17K for OSC, you got ripped off.
OsCommerce is Open Source, meaning it's free to distribute and modify the code if your web developer understands how to use the OSCOMMERCE framework. Thus, it's easy for someone with basic HTML knowledge to tweak the colors, and add your Logo, it really takes someone with experience and full understanding of PHP to give you custom modules, or just provide you with the functionality you are looking for....
I've been using OSCommerce for my clients for the last 10 years since it was OS Commerce release 1.0. Thru the years I've learned to modify the framework, and basically create custom modules, depending on the clients needs. We've added a Bidding module to Oscommerce, Integrated one Login for the Shopping cart, PHPBB Forum, and on top of that I've integrated the same Word Press Login to work with Oscommerce and PHPBB. And yes that was a 10K job because the Logins had to be integrated, we had to custom code the template for the site, and we had to add Flash, and AJAX scripts to blow up the images.
Now, as far as your project goes, try to get your money back, and have one of the fellas here on the forum help you... VWWorld's would be able to help you... or one of the other fellas.... See you are telling us that you are limited with OS Commerce but you haven't told us how you are being limited?
OS Commerce is very powerful, but before you committed to it, you should have made a list of needs and wants... Sometimes OS Commerce is suitable just for almost every product, but there's times where it must be modified to fit your needs.... What exactly are you selling? Digital Media? Downloadable goods? Tangible goods? I know that OS Commerce is not good with Car Parts, since you can't enter Model, Make and Year... In that instance the engine would have to be modified to suit those needs...
Web Developer / Owner / Consultant
When starting your own business the need to "know-how" is greater than money!