As far as being able to handle an active Web sales business, this should not be a concern. Companies such as Ticketmaster and Travelocity make extensive use of MySQL in high-transaction environments and find that MySQL scales just fine for their needs.
As for the choice of model, I can't really answer on what information you gave. I know a lot of organizations first look at moving the back-end database then worry about changing their application down the road, and it often seems that making small changes in progression can be easier than tearing the whole system apart at once.
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