Taken straight from the Paladin forums on my guild website:
Karazhan min stats:
Defense: 490
102.4% total avoidance (w/holy shield active)
HP: 9000 unbuffed
SpellDmg: 150+
Armor: 10k unbuffed
Must have 5 points in Ardent Defender
Highly recommend that you have Improved Holy Shield
These minimums are EASILY attainable without setting foot into a heroic instance or having any level 70 epic crafted gear. They also serve just fine for main tanking on the first few encounters of Kara. Of course, these are just entrance numbers, you'll have to steadily improve them if you want to tank bosses further in.
I personally use this build: http://www.wowhead.com/?talent=sZVhthx0dggqtV0x
It gives the max amount of Stamina, Armor and Defense boost available. If you feel so inclined, you could migrate 3 points out of Reckoning for Shield Specialization.