Since you mentioned your background is in .NET development then I personally (because I am a NET'tie too) would recommend Windows IoT
I would have thought you already have Visual Studio but even you do not you can download the community version for free.
You can develop UWA (Universal Windows Applications) using XAML (really easy layouts) with C#/VB whatever your flavour to do simple things like timers connected to HDMI screens.
Windows IoT supports GPIO pretty well by now. Since you are .NET developer you will also appreciate the ease in debugging code running on the Pi in your visual studio over the network by just running in debug mode.
The nice thing about UWA is that you then run that same application on any other Windows 10 device, like a smart phone. So you could use a smartphone or laptop to control the count down timer all form one application over WiFi.
A great starting point for .NET development on the Pi
If however you choose the route of going with Linux/BSD then I suppose Python would be the easiest way to get into it from .NET - Python works really well and you can also use Visual Studio community to write and deploy Python to raspberry Pi. I am not 100% sure about debugging features? but I have used VisualGDB (paid) but it works nice!