Oh wow, I guess clothes that is being sold is just some widget. Too many people in the tech world simply do not get fashion.
In my experience, most large fashion sites can comfortably run on a Magento instance with a fairly simple architecture, normally a small handful of load balanced PHP instances, with a dual read slave / master database configuration.
Fashion itself, however, is a much different beast, being cool, hip, whatever, it takes a lot of skill - to really understand your target market and sell them something they don't even know they want yet.
Source: I'm a software engineer who has previously worked for www.missguided.co.uk, www.jdsports.co.uk, www.boohoo.com and currently works for www.countryattire.co.uk