Ecommerce - Magento or other, Experts Only

We are a small beauty products business and would like to develop a unique, fairly complex web presence which allows our customers to login and buy our products (e-commerce website). The complexity comes from the fact that all functionality needs to be as automated as possible and there are multiple dynamic layers to the site which influence/change the user experience itself. The site will have 2 types of visitors, members and non members, Whilst pages such as ‘homepage’, ‘products Overview’, ‘contact us’, ‘FAQ’ and ‘knowledge centre’ can be viewed by non members, Only those who are members can enter the members section, which is where one can view stock and make purchases. The nature of our products means that even though two products may be in the same product category, they can be very different visually, this means every single individual product must have its own pictures, product ID number and its own description, which automatically appears in the product description. Individual product descriptions and other product information will be dynamically populated on the site via the Product ID Numbers uploaded. Their format will mean pre-defined paragraphs will apply to the product descriptions. Other elements/functions of the website will also change as a result of the product ID numbers. The website must alter the options/links/offers etc it gives visitors depending on who the visitor is and what the product is. There will be approx. 2000 individual products for sale at any one time and we expect 50 users at any one time, though this is hard to predict. Whilst the site will always be doing a lot of ‘thinking’ in order to make the UEX feel customized, the products and site navigation must be very quick. This is the most vital point. Our direct competition has 8000-12000 hits per day, with 100-120 people requesting membership daily, we expect more, and need the site to be able to handle the processing of this number of customers and ensure they don’t spend too much time waiting for pictures/video/functionality. Our customers are themsleves resellers, who put their own margin on the products and sell to the final customer. We would like to develop the ability for our members to drag and drop items into their personal retail section of the site which can be accessed by their customers. Their end customers can then log onto our website and by using a password given to them by the member for this precise purpose (reseller password) they are directed into a customer facing version on the respective member’s retail page (this section is will be called ‘MyShop’.), the end customer sees the product, and makes the payment. Our member’s commission is added to their redeemable credit, on their member’s homepage. There will be 2-3 different levels of membership, each level giving special privileges to the member (e.g. discounted prices, access to certain site functionality). Membership levels will change automatically according to how many products are bought within certain timeframes. The site needs to automatically grant and automatically withdraw privileges according to these rules. E.g. the withdrawal of certain privileges may mean no access to some website functions. Certain events must trigger automatic emails to members, the content of those emails will need to draw upon data fields of stored information. There must be no ongoing costs therefore the administrator section must be simple to use yet must have the functionality in place to ensure the site is highly customizable. Multiple levels of administrator access will be needed. All non administrator inputted material will require approval. Incorporation of google analytics to the administrator section. Site must link into facebook and twitter, with as much automated updating on facebook as possible. The winning bidder will also ensure complete SEO (on site and off site) for the final solution. There is a lot more additional information, please read!