E-Commerce Solution for Start-Up

I am in the beginning states of a start-up. Researching my e-commerce solution choices now.

Being a start-up my funds are limited, however, I don't want a site that looks amateurish, and there are plenty of them.

I will be working with a web designer/developer, but want something that I can add/edit my products to save on costs down the road.

Thinking of doing a hosted solution initially, then moving to something more robust once the business gets off the ground.

I really would like to use Amazon Checkout for the functionality it provides, so am seriously considering Webstores by Amazon. I understand the limitations in design, however getting something up relatively fast and inexpensively is a plus, as is the benefits of using the checkout. I don't think the commission is out of whack since it includes cc processing, fraud protection, etc. and I know what I would pay for merchant account.

I will be drop-shipping, so inventory management/shipping not an issue. However, I have heard that Amazon does not (or didn't) handle shipping calculation or sales tax which is a concern. I also have read that Amazon does not integrate with Quickbooks, which is a disappointment. What's the solution there?

I'm torn between Amazon webstores and a more customized option that I can have integrated with Amazon Checkout. Some options are OSCommerce (have heard this is old technology, though), Miva Merchant, Magento.

A hosted solution that can integrate is CoreCommerce, but can't find a whole lot on it.

I realize that a nice-looking site that no one can find is useless, so something SEO-friendly is really important, as is something fairly feature-rich so that I don't have a zillion software programs all trying to talk to each other.

Any advice would be appreciated.