From what I understand of your question, it boils down to:
How do I produce an infrastructure that scales using Microsoft tools for the frontend and backend.
You don't have a Stack Overflow question there, yet.
You're still designing, looking for options. You'll have Stack Overflow questions once you've gotten going, and are stuck implementing specific parts of your site/infrastructure.
What are my options for delivering an efficient system and still provide a rich interface the end users are requiring?
This is simply too broad to fit anywhere, in my opinion. It is not really answerable.
How you build products that scale is a topic covered by entire books and scores of blogs and research papers. Whole companies are built around providing products and services for that (and you're trying to get into that market apparently).
Has anyone been involved with a similar project, and if so, is the system you are familiar with running efficiently?
Stack Exchange does Q&A. Stack Exchange sites aren't discussion groups. (Please stop calling them forums, BTW – there isn't a single forum on the Stack Exchange network.) Don't phrase your question like that, you're essentially making it off-topic on all the technology-related SE sites. If you think about it, you're not even interested in direct answers to the above question. Knowing whether some's system somewhere is running smoothly or not doesn't really help you in any way, unless you know exactly how that system is built, which you forgot to ask about.
Programmers does handle software architecture questions, but I don't think your question, as it is, is focused enough to fit there, and "how can I build a web app that scales" has already been asked.
I suggest that you go through the cloud computing, web-development and scalability tags – there's a lot of information out there already. You'll get some great information about a lot of the parts that need to fit together to make your solution scale.
If there are parts of your design that you still have questions about after that, Programmers would be the right target site.
Go to Stack Overflow once you're finished with the white board part, you're sitting in front of your favorite IDE and you're stuck implementing a specific feature/piece of code (or getting your IDE/compiler/SCM to do what you want it to).
In my opinion, something else that would help you right now: start prototyping. Get a demo up and running to get an actual feel about how your current idea of what your software stack needs be actually matches up with reality.
You'll be faced with concrete problems that are easier to reason about than theoretical scaling issues. Programmers can help with these problems on the design side, Stack Overflow can help on the code side. (And potentially Server Fault too, they have quite a few excellent Q&As about capacity planning and load testing, which are issues you'll be faced with.)