After finally catching up on the reading and suggestions this morning, and taking into considering your location @Tomatot, I agree with the recommendation above. Feel free to swap out the HD 600 with the 650, but the 6XX series paired with the iFi ZenDac is a solid recommendation. I’m sure it would sound great driving the Sundara as well.
Another option, if you’re inclined to increase your budget a little, is the newly announced Moondrop Venus planar magnetic headphone. Based on the reviews from the people I actually trust, it’s on par or a little better than the Sundara sound-wise, and it has much better build quality.