As little as I know about PVP, I'm quite sure Arena season 3 is up already (for a month?), and yes, you can buy S1 for honor and BG marks. From what I know it's better for Arena than BG sets. Prices
As for PVE - T6 is a dream you're probably never gonna see in your gameplay. Except maybe for when someone links it in your chat window.
Unfortunatelly, Blizzard made it so only a few % of players can see high-end game content. Shame. :/
Anyway, I'd recommend you to use this planner
http://wow-loot.com/gearguide.phpset on sources:
Include: Faction Rewards
Include: Random World Drops (boe items, buyable off AH)
Include: Heroic Mode Items
Include: BoP Tradeskill Items (if you're a tailor)
and your stats of choice. There are some really good quest rewards on lvl 70, especially in Netherstorm. Also, my mage is only 62 atm, but from what I know, the spellfire set > all untill T5, so if you get that early, you won't have to replace it for a long, looong time.
Btw, care to share your raiding build after today's patch? I'm tempted to try 10/40/11, but I don't really want to drop Arcane Meditation.... :/