honestly, za is several steps up from kara. it would be best if you can to gear up as completely as you can from kara, gruuls and mags. as far as stats go, i'd suggest that for za you prolly want to be getting closer to 28-30% crit and 1800AP. i'd say it's easy to get 2k AP and 30% crit just from kara gear and badge gear. but 2k AP and 30% crit might be more than is NEEDED for za. i just like to aim high personally. a lot of your readiness depends on how you play and your guild. if you play ret properly you're being useful beyond your dps and the minimum stats will be enough for you to be useful. if your guild is well past za and just running it for fun and to gear you up you'll be fine whenever they feel like taking you. if you're just playing for pure dps and your guild is working through ZA you better make sure you're at about 30% crit, 2k AP and hit capped to make sure you're actually contributing enough. it's not hard to get from combining merc glad gear and badge gear. i'm currently working offline on my "ideal" ret set based on gear you can get outside of raids that will be upgraded only in hyjal/bt/swp for those that don't get a lot of raiding options. i'll put that up in my ret guide when it's completed.
and as far as pvp gear in kara, it's not maybe my preference, but ever since S2 was avail for honor merc glad gear is actually better than anything you'll get before kara. most of the merc gear is better than t4 level gear even for pve dps. if you're working on merc glad gear, i'll let you know that the helm, shoulders, chest, neck and ring are on my new ret gear list. get those first. after that get whatever pvp gear you want for pvp use and use it only till you get a better pve peice. don't bother with the pvp weapons, the badge axe is far better and will last a long time. it's the first badge peice i'd recommend getting. huge dps increase.