Hello! My name is Oleksandr, I work as a software developer for more than 1 year and have Master's Degree in Computer Science.
I write mostly in Go, but also have a great knowledge of Python and C++. I used to create applications with GUI for Windows with .Net as well.
So regarding the points:
1. Any platform you wish. As I see, you would like to have Windows, there is no problem.
2. If you want GUI version for Windows, there will be C# then.
3. Both are not a problem.
4. I believe, the worst scenario will be 3 days.
I can make versions for different platforms, but then the price and deadline can be different too.
Best regards,
Oleksandr