So...
1. Working on my Heretek character. Slowly. Researching it carefully. Does rarity matter when taking cybernetic implants to account?
Normally yes, but for the Heretek's two free cybernetics it only specifies rarity not scarcity (that seems like an oversight but I'm not going to correct it (I told you I'd try to make this easy on you)). So you can pick whichever ones you'd like. I will be posting a complete list of the armoury eventually once I get there. But if you're doing your research just post (or PM me) what you're thinking and I'll cross-check it with the rulebook. If it doesn't line up I'll search for the closest thing and run the option by you, if that doesn't work for you I may just rule what you'd like as good and we can go from there (Gah! That's one of the things I love about being the GM, the Power... THE ABSOLUTE POWER!!! THE UNIVERSE IS MINE TO COMMAND
TO CONTROL!!! (Bonus nerd points if you know what movie that's from

)).
So...
2. Not sure if I did the reputation roll properly. I totaled out at 24.
https://imgur.com/a/zXRM3
Did I dice roll properly? I divided by 2..so...5. Then added 19. 24. Am I missing something here?
That was absolutely 100% correct. You're character has a Reputation Score of 24. Nicely rolled!
...can you please explain what the Apostate talents and the commerce skill do?
Black Crusade tried to streamline a lot of the skills, talents, and traits in regards to the three games before it (Rogue Trader, Dark Heresy, and Deathwatch respectively), or so I read somewhere in one of the reviews of the game. Like the Armoury I will be posting a complete list of the Skills, Talents, Traits and Psychic Powers eventually but as for your specific question sure.
COMMERCE (Intelligence)
Commerce is an understanding of wealth, value, and exchange and how to use it to either make coin or procure goods and services for the best price possible. This Skill covers everything to do with deal making and business from bartering over chickens to engineering interstellar trade agreements and ensuring a healthy profit. Commerce also includes an understanding of the value of things and can be used to appraise items or evaluate something's potential worth. It is important to note that this skill is used to get a better price. Acquisition tests are
always rolled using Reputation. However, using this skill to appraise an item's value or haggle with the seller can improve your eventual acquisition roll.
When Commerce is used against an opponent (such as to broker a trade deal or haggle over a piece of gear) it is always an Opposed Test. Commerce opposes and is opposed by either Commerce or Willpower.
Skill Use: One minute on average, longer for complex deals.
As I stated above I will be posting the talents list, unfortunately I'm only halfway through it right now. But it is coming. I promise.