New player question

Hi I just recently started playing warhammer 40k with some of my friends and i got a custodes combat patrol as my first set. I'm a bit unsure on what weapons I should build for my wardens, as the people in my playgroup keep saying that ap -1 should be enough and having -2 won't make a big difference since most units have an invulnerable save and won't be affected.

On top of that i was wondering if buying the new box that was annouced with the new year reveals would be a good way to expand my army towards 2000 points or if i should buy something else to make my army more competitive oriented. Btw i also bought an inquisitor Draxus and big boy Trajann Valoris but I'm thinking i should also get Valerian since his passive seems strong af.

Anyways, thanks for your time and have a nice day