I would run a Boomkin with the DK, for the buff and the heals and the survivability. You have decent DPS and heals on both. The Boomkin can hang back on fights, and while AOE will be harder, you also have knockback and DOTs like fairie fire and insect swarm. You also get a decent group buff as well. Macro some simple Boomkin rotations, and heals, and you should be good to go. I love my Boomkin, I miss her when I run a team without her. Plus, when dual specs come, you can switch from Healer/tank or healer/boomkin or tank/boomkin. Lots of flexibility there! (However, gearing a boomkin takes some research)

If you go Lock, go pure dps for nuking, as your DK should hold aggro and the lock can hang back and nuke from far away, and you can put your DOTs on simple macros. I run a demon/destro lock with my team, and while he's very very good to have, as I don't have a tank (felguard is my tank), a felguard would be a waste with a DK.