This is a subjective question, as how good the healer needs to be depends on how well geared the tank and dps are. The lower the dps of the group the longer the fights take and more healing is required.. the lower the level of the tank's gear the more damage he/she will take and more healing is required. I've been into heroics with raid-geared tanks who barely take a scratch and I can get away with the occasional HoT to keep him topped off.. other times the tank has been in quest blues and each pull is a life-or-death race to keep him alive before the next hit lands.

If you've done all your end-of-level quests for the good rewards, done a fair share of the normal dungeons, and obtained a couple of rep rewards with factions like KirinTor and Wyrmwrest temple, you should be fine to head into heroics. Start with the easier ones, like Nexus, Culling of Strat, Violet Hold etc. And in no time you'll be filling out slots with tasty conquest and valor gear.

My death knight is specced 0/53/18 as a Threat of Thassarian spec, and i was wondering, What hit rating amount for dual wielding should I aim for?
And would I lose dps if attempt to reach the hard cap? (all white attacks and special attacks hit, no misses).

Lastly, what stat should I value most?
I use obliterate as my primary source of damage, supplemented by frost strike and howling blast, so i would assume Armour Penetration then Crit, assuming I've reached the expertise cap and am happy with the amount of hit rating I have.

Would love to have someone help me out with this, TY in advance.