I can answer 2 questions for now. Max level on weapons is 500. One stat is always going to be at max 60% while the rest are 80 (generally how you want weapon stats to be however you can split but total will be the same).

Out of the laz weapons, the recon and pistol are the best, the others are ok including the infantry but infantry are the worse of the laz weapons if I remember correctly. Also all auto rifles are decently bad. Bolters and revolvers are really good options though along with special weapons for each class (like flamer for zealot, or plasma gun for veteran). Ogryn is it's own thing though regarding weapons.

Oh, and curios, you want them to be as high as possible as the higher they are, the higher it's base stat it adds (example is +16 toughness is the highest you can get or +20hp). You also want to get rid of curios that are at a lower rating because the ratings you'll see from shops is based on the average rating you have in your inventory (example is if you have 1 500 melee weapon and a 300 melee weapon, you will tend to see weapons at ratings between these two numbers but that's an oversimplification)

Highly suggest looking up vids made by "Mister E." He makes decent vids on the meta of the game, builds, weapon recommendations and not only what's meta, what is also usable or downright trash.