I bought a set of Stage 8 locking fasteners for the DHM manifold I'm waiting on.
I am not sure how they compare to 8.8 or 10.9 but since the job is only to hold on a manifold, I think the locking feature is the key attribute.
When headers heat cycle, the bolts get stretched and relaxed, stretched and relaxed, again and again. Usually rust will be enough to keep them from loosening but if not, the Stage 8 system will.
If you just want budget bolts, anything as good as the metric equivalent of an SAE grade 5 should do fine.
If you can find it, a 12 point or inverted torx head will be harder to round off than a 6 point head. Use anti-seize if you are worried about having to remove a stuck bolt.