This guide will help you to get your Death Knight ready for Naxxramas 10 and then you can progress on your own from there.
I will suggest the best gear and a couple of alternatives for the item for those who may not have the time or the gold to get the best.
Head
Spiked Titansteel Helm - This helmet is crafted from a Blacksmith. It is very good, has a meta slot aswell as a nice amount of hit.

