Depending on your Death Knight's chosen profession, you will need to stock up on either Potions, Armor or Weaponry from other players, spending your valuable money in the process. If you have a good buffer before you even begin, it'll help you that much more to ease into your new character.

One last thing you should keep in mind is that you should stock up on Runecloth if you want to be in good terms with all of your major cities' factions. This gets overlooked often by level 1 characters since you gain reputation with all of your cities and factions quickly as you level, but with a pre-made level 55, you will be friendly at best. If you want all of the perks which include some discounts and some achievements which include being Exalted with all your city factions, then you'll want to have enough Runecloth to turn-in your way to the top.