Search for a motherboard compatible with the i7-3820 processor.
I plan to use around 16GB of RAM, since 8GB drains quickly when running a web browser and background applications.
If I can't locate a specific Ryzen CPU, I'd consider the 8 Core Xeon and X79 board instead.
These seem like a solid choice considering the cost. Things will likely move quickly. The boards might be tricky, but it should be manageable.
I concur. The X79 boards are quite difficult to locate in my view. For the GPU, I would opt for a Radeon R9 280X.