Sorry for the delay - went out for dinner.
You can DO missions in a team but at present, each kill only gets attributed to one player. And that is the person who did the most damage to it. Which is a bit of a bind, but such is life.
As to cars I've not checked markup recently but I like the Saehrimmer (Landrover) as a vehicle... If you just want to zoom about then the single-seater dune-buggy thing is probably smarter for you, but I like the landy as it has passenger space, can get a weapon mount, etc.
But there's all sorts, including a hovercraft, which might be more practical if you want to only get one vehicle for both land and water. I got a boat and a Landy and a VTOL though