Hello, I have read through your req. doc very thoroughly. First, I and my team can do it. So thats the good news. 2nd. your price of up to 750$ is unrealistic. maybe it is just there. In any case. We have done things like this where we combined the modern looks, landing page with e-commerce functionalities. On top of that you require personal domain for every user. We loved your description and I think we won't have any problems communicating. Usually I offer 3 options: platinum, gold and silver. For you I go with gold or platinum, since you want to be the leader in marketing accounts and lead generation.
So I offer for now 3000-5000$ for 30 days of heavy development work and we add something (both parties agreed) that would be extra. So far the only thing you might want to add is SEO and SMM marketing tools. Also, use lots of API's - tools for marketers, like marketo or percolate.
In any case, please reach out and let me know if the price is affordable for a great site.