Hello,
Welcome to the forum, I would recommend you download any trial versions of the store modules and have a good look through the admin sections.
The active purchase module may cover your needs as it does allow for flexibility in design of the order process and is simple to use, but it will have some other limitations such as ratings / reviews.
Ownership of code - You'll need to see if the source code is available for purchase through each of the modules, you may even be able to approach the original developer of the module for a custom version of their module, usually most developers are open to this type of work.
Catalook and the AspDotNetStoreFront modules are massive modules - trials are avaialble for both of those module. - Other modules which have also been mentioned since the store reviews in are:
SDN storePortal StoreThanks,