So, I've got another question; at the risk of saying "tell me your whole campaign?" what is the overall intention of this group? You have, in my observation, a group of soldiers from a long time ago, at least somewhat new to this time frame, flying around on an ancient-tech filled, fleet flagship, alone, accompanied by some questionable Marines of apparently significant number. Now, unless 1+ of your players are among those Astartes, I'd like to just exclude them from whatever I'm saying; they could, quite easily, get sent off to some other errand, all die, turn on the players, lots of things. Your Captain, though, and his crew, what do they intend to do? For some hypotheticals, I could see them saying "well, we're unknowing of now, and have very little to go on, or make by with; let's go find the AdMech, and get some info", hoping that they have 10,000 years of information, stored in their impeccable data caches, to fill you in. They could find some, and maybe parlay limited access to their vessel, filled with 10,000 year old devices that still work , and the AdMech could pay them in some repairs and history (if you keep them from just steam-rolling you out a ship you might not have the resources to maintain), but then, they have a massive ship, with massive expenses, and little money to fund it, nor any other craft to defnd/support them, as a battleship would usually require, even in the Navy. What sort of stuff will they do, juggling drawing attention to what they have, with attention to what they don't, and...yep, what is their overall objective? Do they want to help the Imperium, in some way? Find about about, and then break Undred-undred Teef!? Become the next Winterscale, and just swim in money, and power? Make the Expanse a Sector? Even looking past the ship, the Space Marines, and whatnot, I'm not sure what end goal you/they hope to accomplish, whether it demands such a ship, or not. I ask because, as I've wrote a story about my silly RT, I've come to learn that, running a dungeon ruin, or two, is fun and fine, and stealing from Lure of the Expanse, Edge of the Abyss, and the Warpstorm Trilogy gave me an excuse to flesh it out, but other than flying through space, wooing women, and occasionally making some money at a rate most Rogue Traders would consider "bad priorities", even I must admit, his story, and that of his dynasty, is a bit weak, apart from several "episodes" he has taken part in, and later repelling the Tau from incurring into the Expanse. What are his (dynasty's) goals in the Koronus Expanse? i don't always really know.
I don't even want to just look like I'm beating you over the head with your ship; I'm just curious about some of what you plan to have them be doing with it. It can be good to have some story ready to go, before you sit down to pen it, and then come up short, sort of like I have, from time to time, with Qel-Drake and Co. They've got a nice, pimped cruiser, in my opinion, if not maybe really optimized for cheese, and several quite competent specialists, "a person for every role", and I've made them to have taken part in some interesting stuff, but other than sating my desire to put to paper some stray ideas, well... Hopefully, you might have some better luck on that front, so it doesn't just appear as a group starting with a pimped battleship (might never need a "better ship", unless an escort), Space Marine allies, all the ancient munitions a ship like that would carry (plasma weapons, melta weapons, power weapons, and ammo/resources), and whatever else they might have had to otherwise rack up PF to get, or role-play to acquire. I'm not just trying to point a finger; just hopefully help to make it all make a bit more sense, maybe both for you, and those of us who read it, and maybe jumped the gun as to where it was leading, or saw a ship even 100+ PF Winterscale couldn't afford, with his, and 2,000+ years worth of prior Rogue Traders' considerable efforts. Okay, I'm done preaching, and babbling. Sorry about that.