Im surprised you've gotten this far with Drain Hunting, a lot of us gave up on the Voidwalker back in the mid-40s (not that he doesn't still have some uses.)
What you'll find is that once you hit 58, and get into Outlands the gear selection will change a lot.
You wont have to choose between Sta/Int/Dmg, now it'll be how much of each.
My suggestions is that you start hitting your AH whenever possible, and collecting Green "____ of the Sorcerer" gear, especially for Rings/Neck, although you're safe getting pieces for Helm, Belt, Legs, Boots, Gloves, and Bracers as well. You may also get a Wand, Staff, Robe, and back although you'll find you can replace them quickly with Quest rewards. On my server, on my way up, I scooped up whatever I could find -- and it was usually cheap (5-10g), the main problem was finding the pieces for sale in the AH.
You'll find that even compaired to quest drops (which are plentiful and quite amazing), for Affliction spec you're best bet is still often green "of the Sorcerer" gear.
As you grind through your 60s you'll find little use for the infamous +SpellHit required to put out damage in the higher-end game. For now, stick with the basics - STA and Spell Damage, Int never hurt either. Obviously if you plan on Speccing Destruction then perhaps switch some gear over to "of the Invoker" for the Crit%, but for grinding I'd still stick with Affliction for now.
You'll find yourself reaching amazing new heights in HP, MP, and Spelldamage levels by the time you're 60 - its a nice feeling. When you hop into 5mans in outlands you'll notice a few things being different, namely the difficulty of the Instances being upped to match the gear which generally results in requirements for Crowd-Control and focused-fire. In this reguard, in order for you to stay up to par with other DPS classes you'll need to Spam Shadow Bolts and I suggest using an Imp as a mana battery so you do not run low on mana. As for dotting, remember that Corruption has a chance to proc Nightfall, and Immolation is your best use of time, so try to keep those DoTs up, but I'd drop the rest. Save the rest of the DoTs for the bosses... you'll need them there.