Build an interface (API) for MarketPress to receive and present third party data

We're about to build a shopping cart for parts. It's going to contain hundreds of thousands of SKU's and will present parts breakdown diagrams of the different pieces. All of this data comes from a third party.

We've been leaning toward using a Magento cart because it's a proven workflow. The API to bring in the third party data is something we can buy from another distributor who's already created a magento shopping cart and rework the code in pretty short order to work with our site.

Recently, we discussed breaking the mold and using wordpress multisite and MarketPress and the conversation turned to building the e-commerce on the wordpress architecture. Questions immediately jumped up like, "Can Marketpress handle that many SKUs? How would we build the API? This idea is insane. Why try?" So, I thought I'd turn to the experts and ask if it could be done? Can Marketpress Handle the job? At what point would the experience become unstable? What would you need to know in order to build the API between Marketpress and the third party feed?

Would the staff at wpmudev be interested in building the API? Most pros that I've spoken with in the Magento Partner space quoted us about 8 hours to build it for Magento from scratch.