It's in a system without an outfitter
Oops, no, I think that one is hosed.
Capturing without engines or maneuver or reactor and no outfitter or spaceyard? I think that's a no-go. If you have any kind of outfitter, even one without the parts you need, you can refit it by selling the required refit parts to the outfitter and then re-purchasing them for the other ship.
But if you've got no outfitter at all? I think your stuck.
haven't found any story arcs yet
The main mission arc is for the Free Worlds. Surely if you're that far you've already seen news of their independence and such.
Always check spaceports when you land, that is where most of the mission triggers are. And I think the FW stuff triggers in their own space, which is in the south. You can see who owns a system by clicking on the owner name in the galaxy map, then it will colorize the map based on ownership.
The map is actually pretty impressive, I didn't realize it had all these little tricks in it. You can click on basically any word and get specific information displayed.
that 24m heavy warship with the 500 cargo, tons of bunks, tons of hp. It's pretty overpowered
In some ways, for sure. There are better purpose built ships, though. Other dedicated warships are stronger at fighting. Dedicated people movers are better at boarding actions. I think the low end freighter has the best cost (and crew) to cargo ratio, but unless you want like a hundred of the things then the giant super freighter thing with 600 cargo is a better dedicated hauler for trade runs.
That said, the Bactrain is certainly my primary. And those other ships (mule) by the same manufacturer make up my secondaries. The mules might be sub optimal as escorts though, there is no benefit to them having large crews or much passenger capacity of their own. I've never seen a mission to move over 100 people.
Of course, even though I have it kitted out with atomics and advanced weapons, I still get creamed in some of the larger scripted battles. So, it's not that OP. If I was running a fleet of 5 or 10 dedicated warships, I'd probably do a lot better.