Ya I would recommend a DK main as I can see you getting kited pretty well.
I`m actually thinking of adding a 5th paladin to the team instead of the DK. I have no fun boxing my ret team with a dk as main, I really HATE dk`s

+ so far I din`t see much advantages. I never had real problems with people kiting me, unless it were really good mages, tho I`m not that skilled with my pallies yet. I think I`ll see how it goes with 5 of them.

Playing the team should be fun anyway, with a dk as main it doesn`t feel like playing rets anymore. I rather select the skills I want to use rather then have them on automode macro.