From the Wiki:
"To comfortably run Naxx, you want the following un-buffed:
+1500 spell power
25% Holy crit
17k mana
13k health"
I'd say that 1300 SP and 15k mana should be fine. As for crit, the 25% is still a good mark to shoot for, especially since you get so much from subspeccing Ret - there's no need for a Divine Guardian build that early.
Unfortunately, with the Illumination nerf coming the other way up the tunnel, we don't know how things are going to shake out. It's quite possible that crit/haste will still be good at heroic/Naxx-10 level while mp5/haste is desirable for Ulduar-25/hardmodes.
Probably best to go for a balance - just grab the best upgrades you can for now - and see what happens.