Even with worse gear than yours you can tank naxx without any problems.
Around 25-27k HP, and being a dk around 50% avoid (infight.. with the +10% Parry buff active) and crit immunity of course is enough for the 10 Player version.
25Players is possible, but you'll be hard to heal and your healers wont like you right now. But a few runs Naxx Normal will help you out, and you will be able to tank whatever you want in the current content. easily :-)