Four toons at 71st, can take many single 80's.
You can contribute to world pvp at any level, especially with four of you.

I'd definitely go with the 2 min trinket as your first pvp purchase, unless you're human and have that as a racial.

Depending how geared you were, there are Northrend upgrades for the leveling up process.
One of spacegirls (shammy) was a mix of T5 and T6, her first up was at 77th.
The rest of my spacegirls leveled to 70 basically at about WotLK release, so didn't have a bunch of gear and were upped constantly.

I'd personally do Howling Fjord or Borean Tundra, and then the other.
Progress to Dragonblight or Grizzly Hills and then the other.
That way, if a quest is really annoying, you can skip it and not be too bad off, even if you miss anything else in its chain.
The Icecrown quests you'll want to do them all, even if they're painful, because they open up dailies which are very lucrative.

Battleground whenever you feel like it, as a distraction or change of pace.
Most of the gear is 30-40k or so.
Try not to go too far beyond the honor cap, since that's basically wasting honor.
The combo of Strand, Alterac, Arathi, Warsong and Eye marks, can be turned in for honor too.
Prior to 80, I'd just hold onto the marks, until you have 100 of each.
Wintergrasp is worth doing at every opportunity for honor.
At 80, its worthwhile to arena at least 10 games a week, even if you just dance naked and die fast.
The arena pts + honor pts is basically a very cheap piece per week.