If I buy this second hand which achievements will I be unable to obtain?
the first post makes it seem like a still have access to multiplayer, but can't level up preventing certain unlocks. It doesn't specify which ones. Modfather and the number plate ones I'm guessing.
Games With Gold for dummies
Mutual assistance for the following games' achievements:
Need For Speed: Hot Pursuit (MP including the DLC),
Gears Of War 1 (Hardcore Campaign, Dom run one of them),
Battleblock Theater (Open for trading, 3 out of 10! Co-op, etc.),
Might & Magic: A Clash Of Heroes (MP stuff),