An arduino or arduino clone still seems like the best bet to me, in my amateur opinion. Nowadays you can get an UNO clone for $4, and that gets you pin compatibility with Arduino shields, plus loads of guides and help interfacing with other components. I still have a Freaduino Micro (besides my original Duemilanove) and it works fine.

And the Arduino can be programmed in C; in fact, the "Arduino language" is just C (or C++ nowadays?) with some preprocessing and a bunch of helper libraries.