My guild on my old server pre-bc killed everything except Kel'Thuzad before BC came out, so I got some experience there.
I would recommend getting lvl 70 resist gear instead of lvl 60 gear if you are lvl 70. For fire you can get 200 resist for 100 badges for example.
Also good is the "of <element> protection" greens at 69-70 etc, where you get good resist and also a fair amount of stamina. If you are planning to do these resist gear heavy instances I suggest to start looking around the AH for such gear. However if you do go in as lvl 70 you can also take into account that you are higher lvl and will resist more. Though you will prolly still need onyxia scale cloaks for BWL.