Ooh as a priest branch-line class, you will most likely not have a huge amount of natural defence to fall back on. Below I reccomend looking at damage reduction also as a possible option, because a 50 sage has access to many means of physical damage reduction. While it doesn't benefit your health amount, it increases your survivability.
So instead, atleast while I played sage I did a few things:
Staff-sage specific option:
- Damage reduction mastery 'Giant Ape Blessing': You are the only class to have a physical damage reduction mastery at 45. If you can invest the time to get this to level 10, you will take a static -10% less physical damage.
- Equipment mastery: You can use the Equipment leveling system to increase your VIT and thereby your defence.
- Prefix: Yes, ideally you should make the 51 set at your level, for it gives great cast time reduction and it is the best armor considering your situation. Prefixes that would benefit what you are looking for would be Vitality and Blessed.
There is one more option, but it is specifically for when you are equipped with a staff. That is getting 'Staff Shielding - 3' from the Magic academy. It will gives you -18 Physical damage taken. This means you will have to need to forage lots of Immortal Ginseng Roots at 50 and hand them in till you have 9000 Magic Academy fame.
On a final note, don't forget to get accessories, armor and a cape with VIT on them, they help too!