Exactly as the title states, I am interested in leveling a DPS death knight and wanted to get heirloom for him. I picked up shoulders/chest. Assuming I'll go for a DPS spec, what weapon(s) (if it isn't one 2H weapon?) should I pick up for heirloom?
There really aren't too many options.
For a dual-wielding DPS DK, you can either dual-wield two Venerable Mass of McGowan, or wield it in your off-hand and Venerable Dal'Rend's Sacred Charge in your main hand. As of Patch 4.3, it's really only viable for frost DKs to dual-wield. Since the haste stat is slightly more valuable than crit (as haste decreases weapon speed meaning more killing machine proc as well as the rune refresh speed), it's probably more advisable for you to go for the first option when dual-wielding. Neither weapon provides any strength unfortunately.
For a 2H DPS DK, the only option is Bloodied Arcanite Reaper. You can probably use it up to level 80, or more if you don't get a better weapon by then.