I have some experience in parallel programming (MPI and OpenMP), and I am planning to dive into GPU programming as well. I do scientific computing, mainly stuff with double precision floating point operations. I am especially attracted by OpenACC (but I will likely explore CUDA Fortran as well). The point is that I have no experience with graphic cards - not even for gaming - therefore I really need some help.
My present platform is: Asus Z170 Deluxe + Intel i7-6700 @ 3.40GHz (Skylake) / 32 GB RAM.
I only use GNU/Linux (at the moment, Linux Mint 18.1).
I do not plan to become a gamer or to do video editing, so I would use the graphic card basically only for scientific computing (for the rest of my activities, the integrated Intel graphics that I am using now would be just fine).
So my question is: What would you recommend to buy? If it makes sense, I would try not to go beyond ~300 euro, more or less.