What does 2nd choice mean?
Do they already have defects then?
The boards were sometimes "nibbled" on short sections or had defects in the first/last cm of the plank – you just use them as end pieces. But only a few planks were like that, which is why you buy one more package than with 1st choice; otherwise, I haven't noticed anything.
And if you're worried about defects, an oiled floor in "gespachtelt rustikal" (spackled rustic) is really ideal. Sanding or filling and then applying a little oil at the end, done. The more uniform the floor is, the more difficult that becomes, and doing touch-ups in DIY always shows a difference.
You have to like rustic style, of course, but it is also a very honest, low-maintenance, and lively floor.
Oiled simply means vacuuming and applying a soap solution damp every few years.
Is a wear layer of 2.5mm sufficient?
A bit tight for sanding. On the other hand, hardly anyone does that anymore, especially not with floating installation.