Although either Dead Money or Honest Hearts can be done first it's best to do the rest in order. It also helps to explore most of the main game first as some of its people and places tie into the DLC.
Spoiler Dead Money is best done without activating HELIOS One until Veronica has helped you retrieve the Rangefinder, because that path has a lot of foreshadowing about Elijah. Honest Hearts makes more sense if you've already explored Vault 22 and have quizzed Caesar's Legion about the Burned Man.
Old World Blues must be done after Dead Money otherwise your Courier will ask a lot of stupid questions during Dead Money, and, as already pointed out, Lonesome Road is the big finish of the DLC story arc.
And don't worry about level requirements, just bump up the skills you've used most often so far and toughen up to the point where you can take hits.