Ok, well lets get a few things straight here, if you aim for never missing a spell or attack on a level 84 target, you're going to be losing too much stackable secondary stats.
As far as best use of gear goes, you'll want to stalk the AH for level 80 Cata Greens/Blues, I personally on all my 80's use the gear that gives ''Stamina/Base stat of Class (Agil, STR etc)/Crit rating. These I feel provide the best source of damage, rather than stacking Mastery for instance. My shadow priest alone has 39k HP, 50% Crit, and 37% haste, unbuffed. These are HUGE stats for such a low level character, take advatage of it.
Be aware though, people know what gear is good for level 80 twinks, so the good Greens and blues, you will be paying a shitload for from the AH. Also, if you have friends, try to get them to run you through the 2 Cataclysm normal intro dungeons, they provide excellent gear, useable at level 80 (Ilvl 308 I think)
Try to snag a cheap Spell power Cataclysm wep for 80's on the AH, they give about a 600 Spell Power increase on WOTLK BiS caster weapons, and are your best choice till you run normals.
thats me atm working on cata blues.... keep losing the others done countless runs today. i thought mastery would be good, but i do love my haste, and i guess crit for mage wouldn't be bad either