Back in the day (pre-BC), a 60 was good enough for boosting up through Scarlet Monastery. Of course, a DK starts with very nice gear for a pre-BC character so he might work up through ZF.

Of course, if you're boosting a group with healers, you can keep right on going up through ZF to BRD to LBRS, right until you catch up with the DK and start in on BC instances.

A 5x DK team would be a blast though and might be worth it if you really want to get money going. On the other hand, lower level mats sell for a ton, so as long as you have an enchanter in your group, you should be able to make some good money while boosting.