Big Valerian sale just went up too.
It starts pretty rough, but i'd say 6-10 are a damn fine arc of comics (6, 7, 8 are basically each self-contained, and then 9 & 10 is a two part-er and generally considered the high point of the whole series). You could just do 9-10 alone, given the plot is pretty self-contained, but it helps to know the characters and see them develop over the previous books and makes those two more powerful.
At the same time, given the elevated prices on euro books, i'm not sure i'd recommend going all the way back to book 1. 1 & 2 especially are *rough*, and you can see them getting their feet underneath them and figuring it out in books 3-5 but they're not really necessary.