A startup should choose .NET right now. With the things that are available in .NET and especially after going opensource, .NET is the framework for building all kinds of application. With the help of Xamarin, you can easily build mobile apps, with the help of WPF, you can easily build desktop apps, and with the help of ASP.NET and WebApi, you can build all your Web apps. All of these is possible with the best IDE in the world i.e. Visual Studio and only one language , i.e. C#. So a start up can have one man army do everything for them(As long as that person has mastered the concepts of .NET Framework and C#). No kidding!!!

I don't think it would be too difficult. If you have a background in OOPS concept, you should be able to handle it in no time. Micosoft makes it so easy for new comers to learn their tools and this is no exception either. So if you are thinking about going to .NET side of the world , welcome!!!

If you are not satisfied with what Visual Studio provides, you can try this tool. This is not a free tool, but a great one. Not only this helps you in modeling your systems but also it lets you visualize your applications from the very beginning. Enterprise Architect Tool

If you would like your users to be engaged in your app and want them to stay in your site for a while, you might not want to build an ecommerce as SPAs. In my opinion, most of the e-commerce website visitors today are used to huge amount of pages and therefore would be comfortable looking at the websites that give them an illusion that they are in the real shopping mode. Most of the users on these websites are just ordinary people trying to buy or look at the products that you have and for them, more than the website itself , price is important. For example, look at Amazon , I don't think they care much about the UI or how the button looks on their websites. But still they are able to capture what people are thinking about when they visit amazon websites. This might be funny but at one point I was shopping in Amazon so hard that I had at least 50-60 tabs open with varieties of items on each page.

I am currently doing that right now! :) Building a little card game just using plain old HTML,CSS,and traditional JS.

Well, I think it is the great feeling of accomplishment/achievements when you implement something cool.