As a tutorial series, I think using plain C is a cool approach and will help illuminate what's going on under the hood and what parts of the engine are really essential. But if you yourself want to write your own game with just you or a small team of like-minded people, I would highly encourage using C++. As long as you don't have too many cooks arguing about which C++ features to throw into the pot, you can pick a s…
I say this partly in jest, but once I have wrapped all my primitives in structs C has a perfectly reasonable rule for implicit type conversions: "don't".