Away from my books but:
Technically, Arcane Recovery is only supposed to function once per 24 hour period, not sure how they'll implement though. Plus x buffs stack with anything. As far as armor class, only different means of calculating base armor class don't stack, only the best is used.
Questions:
1. To get Polearm Master's benefit you must take the Attack Action. An AoO does not use the Attack Action, is defined as a single attack, and expends your Reaction.
2. You only have one Reaction per round, which you regain at the start of your turn.
3. You are wielding a weapon with the Two-Handed property. As far wielding a weapon with the Two-Handed property with a shield, I'm thinking a DM isn't going to go along with that (Though it looks like the Quarterstaff is Versatile).
Thanks
So it means all buffs stack.
The players handbook does not say anything about stacking effects.
Last DnD like game was pathfinder Kingmaker, where you had to look at every buff if it is an enhancement, morale, luck or whatever bonus.
So far I think I like 5E more than previous editions, but I have not played it so far.
Both quarterstaff and spear have the versatile quality. That was the whole idea behind this char. You attack and stop enemies when they get close to you while having a very high defense.
Once again:
Does it make sense to have a specialized tank in the party or is it better to maximize the damage?Damage is also a form of defense because dead enemies do not attack you.
I want to use a martial char, no pure caster. But having some spells is fine.
So I want to have an extra attack, martial weapons and do some damage.
After some more thinking the best idea I had was a dragonborn (fire) paladin (devotion) 7/barbarien (berserker)3
At the end I would have 4 attacks per round (2 base, +1 frenzy, +1 haste), great weapon master (attack again on kill or crit), resistence to one element (fire is the most commen, I guess) and physical damage (when rage), a few buffs and heals, extra damage with smite, immunity to diease and charm, better saves, reckless attack and sacred weapon.
Downsides: MAD (multi ability dependent, need str, con, dex and cha), only medium armor (if I want to rage), only 1 feat or stat boost.
It seems very hard to get very high defense in 5E.
If I really want a good tank I need heavy armor, shield and magic buffs, and the best ones are self only for wizard/sorcerer.
A single class eldritch knight would probably be the best thing if you get to lv20.
I chose battle master/wizard to have the same spells as an eldritch knight plus battle master maneuvers at lv 10, but if you level up further an EK would get more attacks and more feats/stat boost.
Prof. Dr. Dr. Mad S. Tist 
