Depending on how remote their station is... I would say they should have some sort of psychological training to help them deal with the isolation.
Something like that could have a big impact on the type of character.
Also, you mentioned that they are there to "enforce" earths goal. While realistic as an organization (say... the military) I don't think this is something that can be directly worked into the marine template. It's more of an officer thing, not frontline officer mind you. Perhaps the colonel on station has a directive from earth that he has to enforce, after all foot soldiers are just tools at the end of the day.
Dark Angels 1750 win/loss Record
Win - 23 (Belial helped wreck Ryan's puny Eldar)
Loss - 13 (Brice's Necrons wiped me)
Draw - 2 (The 13th hold out in alpha take and hold)