Having tanked with my warrior and my paladin... yeah go pally, much easier rotation and better buffs. DKs are nice but more complex. You can always swap tanks around to spice things up.

As for focus-less, target-less leader-less setup versus main - followers setup. I think it really depends on what you want to accomplish in the game, which hardware/software your using, and how complex you want to make your setup. There are many threads on DB.com that explain how to do both. Your concern about click based healing is valid, when I want more control over my priest I mouse over to that window and use healbot for more granular heals. I have some basic tank and aoe heals tied to key mappings. The ranged targeting leader idea is going to be rough as your tank is going to be the most mobile toon in your team in instances, unless your running all pallies or all dks (multi-tank boxing!)

Good luck with your boxing!