I have PVM knight. From what i saw you dont need any CON at all as monsters has no dodge.
I have aprox
230 sen (seems need 200+ to crit most of hits vs mobs)
rest all DEX for better block amount
As it was pointed out before, DEX helps alot.
You can use luna or hunter set, both works relatively fine (first one is easier/cheaper to get).
You can use valor damage gems on Head/etc, for shield i would recommend block rate (4.5%) gem, secondary option would be damage reduction.
+15 level shield has more effect than +15 armor and low shield due to knight passive and mechanics.
I tested out and found that +11/12 reinforced gear with +15 shield is more tanky than +15 reinforced and +11 shield for example... many things is best to try out rather than just look numbers.
In current patch any defense over 7.5-8k becomes less practical as damage reduction from raising it is practically none (look defense "damage reduction" and try to shift items), better put more into skill power or any other damage items.
Offensive gems/etc depends more on what weapon (xbow, blunt, sword, etc) you use ... personally i prefer AOE as it gives you extra taunt and make significant damage to monsters stacked around you...
Charm doesnot help much as most of your skills doesnot have any PVM useful debuffs...
If you learn all AOE's, Taunts and shield passives (except any attack speed/accuracy weapon passive) you will do well in PVM as you can tank alot (and passive boosts your attack power/regeneration on tanking time).
I have tried exalted knight and also brave gear out in dungeons, both works fine (exalted has higher block rate though).
It really depends how you play and how good cleric is with you, also if group has some high damage makers aside of you.
There is no "best/optimal build" for anything in this game, it all comes to your own and your group game style and abilities.
Anyways, good luck and i hopefully i didn't make things more confusing for you now