Disclaimer: I work for a companycompany which produces UDP file transfer software, probably a competitor of the maker of the UDP plug-in you talk about.
Yes it is possible to build a UDP file transfer application using C#, but no it is not possible to do it well within 240 developer hours.
UDP/IP involves placing nearly raw datagrams onto the network wire. Things you take for granted with TCP/IP, like making sure the datagram gets there, avoiding duplicates, keeping messages in order, making sure you don't send data too fast or too slow, all have to be done by you.
UDP based file transfer solutions are becoming very popular because of the opportunity to develop all of the transport functionality from scratch, thereby avoiding a lot of the problems inherent in TCP. UDP is simply a blank slate upon which one can create a better (or worse) solution.
Throttling simply means making sure that the data rate does not exceed some set value. That's actually pretty easy by itself. All UDP transport solutions that I know of will do that.
Making sure that the network can actually handle going that fast is much harder and is a key differentiator amongst the various UDP solutions out there. Cramming too much data into a network path, even for a moment, can create huge disruptions. Setting a throttle limit is the simplest way to avoid that, but it only works if you know in advance what the network capacity happens to be.