We just had someone from EFI come in and walk us through their online portal, I was pretty impressed, really hoping we get it. You can read more about it here:
EFI OPS - Advanced Web-Based Solutions. They had mentioned that they offer bare-bones packages that you can add modules to as needed, so it might be right up your alley. They also offer unlimited customizable storefronts so you can have one for each of your bigger customers if you wanted. We haven't gotten it yet so I can't really fully endorse it at the moment, but it looks great from what I was shown.
I would definitely NOT recommend Kodak's Insite Storefront. There's very little customization, it's not very user friendly, you need a seperate add-on for Illustrator called Darwin if you want to do VDP, and you're limited to only a few storefronts unless you want to purchase licenses for additional ones. EFI having unlimited storefronts was a major selling point for us, since a few times a year we get a potential new customer that wants to see how everything works before committing to us.