Hello,
I am a skilled C# developer and I have already done projects similar to what you ask for.
For a given .csv file I will output to the console, or to any other medium you would like, and output the median and mode(s).
The algorithm I would use would work very fast even for large amounts of data. It would also be very memory efficient.
As I've mentioned before, I have already done projects very similar to this one. For about 3 months I was working on an enterprise application that handled Excel - Csv parsing and solved cross-compatibility issues so I am fully familiar with the Excel .csv (even some finer detail which I think you won't need, but might come in handy).
I write very concise, very modular code and stick to the most modern programming practices.
I am fluent in English and available on Skype if needed.
I can guarantee high quality service and fast delivery. I can finish this project in under a single day, a full day at worst.