[eluser]IgnitedCoder[/eluser]
ExpressionEngine is good for what it does and you could of course build out your product with it... That said. Make a list of the features you have for your project, nothing complex just make a list, then match each feature to a addon EE can provide. Once you've done that consider and think about whether you like to know whats inside the black box.
Most EE addons IMO are a black box and since I prefer to know exactly whats going on in my code I tend to steer clear of EE projects unless a clients requests it. However EE is built on CI so that might be moot at this point. Bottom line is you could build it with EE if you wanted to but you have to figure out how much control you would like to have over function vs form.
EE is a pre-built CMS and that means pre-defined rules whereas CI allows you a greater freedom to experiment. CI also means more libraries from the community many of which you can alter to fit your needs. Find sites that boast millions of hits running EE and ask them what they did, how they setup their environment etc. Checkout ShopIgniter its on of the biggest CI projects I've seen.
Hope this helps some.