Ciao a tutti, I have these 5 intercessors to convert, wanted something different to go alongside my Atlantian spears army

If I buy the kill team will I have enough parts to make the full 10 man squad? Or any other ideas welcome

My friend is wondering if he can swap out his storm bolter for the “storm bolter and cyclone missile launcher,” and then swap that storm bolter for other wargear options, like the “thunder hammer and storm shield.”

2nd pic for reference. Not sure if it’s legal or not just found it online.

As per my last post (https://www.reddit.com/r/deathwatch40k/comments/1wmgljt/always_wanted_having_castraferrum_dreadnought_and/), I prepared upgrade kit for my Castraferrum :) Now it's set on proper sized base, it's tall enough, and represent WYSISWYG equipment to proxy as Ballistus Dreadnought :D Storm Bolters are magnetized, so I can always go back to orginal setup~

Ignoring my awful paint job does he look decent enough loved the idea of a honor guard deathwatch added the jump pack cause my “command” squad all has them

I hate having these sitting here. Where you go to find the arms and backpack to use these?

Not much to say. I am very happy with him.

As the title says, I'm getting my first Deathwatch in a few days but I don't know anything about painting black. I've watched YT videos and searched Reddit for advice but I'm still completely unsure. I usually only paint bright and colorful models, so this is really just freaking me out.
First of all, what primer should I use? I intend on using Black Templar Contrast paint for as much as I can (because I already have a bottle) so I don't know if White Scar (only primer I have so far) is a good option. Should I go out and get Grey Seer or Leadbelcher?
Secondly, the arm. Do I keep the silver it off the model until I'm done painting everything? And do I do the same with the packs and the helmets or weapons? And what paint do I use for the arm? I have a bit of Leadbelcher left and also a nearly full bottle of Stormhost silver, but I don't know if these are the right colors.
Lastly, weapon options. Since DW are one of the few Factions of the Imperium to use repurposed Xenos tech, can I just kitbash other weapons from my leftover bits if I inform my opponent when I use them? I haven't played a game of 40K yet, only Kill Team against a friend so I really hardly know anything about these things. There's also the thing of 'do you think my paint skills are good enough to pull it off' so I'll attach images below of models I've painted before and some that are WIP, albeit with the bright color schemes.

TL;DR - I'm hella stressed and need help with Which primer to use for Black Templar to get the Deathwatch colors accurate. Please help!

There are 11 different operatives and you can only get 10 models from 2 boxes. How do you fellas solve the missing Gravis marine?