Unfortunately, if you haven't bought any BoE gear by this point, it's probably been bought off the Auction House and either hoarded or put back up at an insane price by now. Beyond that, you don't technically need the gear. You'll get some good upgrades soon, and the base Death Knight quest gear will get you through Outland just fine. Still, if you want to look for one thing, see if you can find a cheap Blade of Misfortune. It's a nice upgrade over your base runeblade.
Consumables and Tradeskill stuff
Major and Super Healing Potions: So, if you gathered Arakkoa feathers we mentioned earlier, you'll probably get a bunch of these from the turn-ins, which is good. In addition, you can farm up some Noth's Special Brew in the starting area. If you aren't collecting feathers and don't plan to farm the brew, though, buying some health potions for emergencies isn't a bad idea.
Bandages and First Aid: Now honestly, I haven't pre-gathered cloth for leveling First Aid. I found that since I started with around 275 First Aid and got plenty of Runecloth and Netherweave while leveling, I was easily ready to churn out the frostweave at level 70. Still, If you have tons of extra Netherweave from dailies hoarded on a bank alt somewhere, it can't hurt to send it over.
Food: If you've been hoarding meat from the cooking dailies, now's the time to put it use. Death Knights do their damage, even their spell damage, off of AP, which they get from strength. So check your meat supplies, or check what's cheap on the AH, or just go on a killing spree in Nagrand, and bring on the meatfest. Ravager Dogs and Roasted Clefthoof are especially good for a growing Death Knight.
Elixirs: Here's another thing I'm not worried about. It'd probably take a lot of money to keep my Death Knight in Elixirs for his entire 55-70 grind. Still, if you have the extra herbs or money lying around, it can't hurt, and it will be an edge. Elixir of Major Strength and Elixir of Major Fortitude would probably be the best for Death Knighting
Adamantite Sharpening Stones: While they will go obsolete and be unappliable to level 71+ gear, until then, your gear can still use them. It's one more edge in the leveling process. They're also pretty simple to make: 2 Motes of Earth and 1 Adamantite Bar. I highlighted sharpening stones if only because my experience as a Death Knight in Beta was that every single good weapon upgrade I saw until level 71 was bladed. If your mileage varies, you may want to look for Weightstones.
Armor enchantments: The basic Clefthoof and Cobrahide leg armors are usable at 60, which could be worth it for a quick boost through Outland. If you can't afford or find the materials for those, you can also stick with the Heavy Knothide Armor Kit -- and you might want those for a quick and cheap enhancement on your other applicable armor slots anyway.
Glyphs: Which Glyphs you get for your Death Knight will probably depend largely on your spec and personal preferences, but here's a few suggestions. If you're Unholy, you'll most definitely want to get the Glyph of Bone Shield, and the Glyph of the Ghoul will likely be a good investment as well. For Blood and Frost, getting the glyphs of Obliterate and Death Strike will probably be a solid choice to start off.
If you haven't rounded up some bags for your Death Knight yet, don't worry, you don't need to panic too much. You'll get a nice set of 12-slot bags that should last you for at least the newbie quests. However, you do have a few options if you want larger bags. If you're really rich, you can just throw a few Gigantiques after your Death Knight and be done with it. If not, you can see if you can scrape together the material for an Imbued Netherweave Bag (You can grab the silk from Spiders in Terrokar Forest or CoT:Black Morass), or even just some plain old Netherweave Bags.
For the record, a bolt of Imbued Netherweave is made with 3 Bolts of Netherweave and 2 Arcane Dust. A Bolt of Netherweave is 5 Netherweave Cloth.
Get on your Marks!
So once you have this stuff, you can luckily mail it to your Death Knight right away. As soon as you're out of the Ebon Hold and getting ready to attack the Scarlet Crusade, you can head down the hill to a ruined village with a fully functioning mailbox. Now if only zombies had banks. I mean, besides the Forsaken. Who I guess aren't technically Zombies, per se.
Anyhow, good luck with your last minute shopping, and we will see you in Ebon Hold!