Well, Banuk is legitimately not worth more the 2500xp. It's literally <3mins to complete.
That's Dead Orbit's shotty- great base stats but it's randomly rolled so it can vary wildly. Won't necessarily have full auto, for example (unless that's built in? don't think so though).
If you're collecting Hand Cannons, the Fixer is a crucial buy. It's the de facto PvP legendary HC, imo. And the built-in Radar while ADS, which I can't get on Hawkmoon without Knucklehead, opens up all kinds of other builds.
Final Boss is a relatively superb sniper though. Fantastic impact and rate of fire. So if you're missing those in general it's the better use of marks.