@Joshua, I'm currently looking at developing my own ecommerce platform as part of something much bigger. It would be nice to have a chat sometime to see what your requirements are and what typical requirements are for ecommerce platforms. Seeing as you've already mentioned 300k products with 1000s of options each, that would be a good base for me start from. If I can cover all your requirements, then I would probably cover most requirements.
Magento from what I've seen is very complex to use, very complex to develop on and their database diagram printed in the smallest fonts I can read on the biggest paper I can have it printed on spans an area bigger than my whiteboard.
Let's have a chat sometime ...