Never go priest + sadhu, oob does not work with blessing nor sacrament.
ps. put 1 point into sacrament for the additional auto attack hit.
As for the op:
Healing and damage prevention is expected regardless of the build.
Anything else however nice it might be is frankly optional.
Stats are also irrelevant. The difference between a str build and a int build that fulfill a healing role is 200-300 ticks usually. Not remotely enough for it to be a concern when health will easily surpass 4k past lvl 100.
Circles/classes matter a lot obviously, as they determine the defensive and heal skills you’ll have.
Useful skills for a healer role:
Cleric:
-Heal, duh. needs to be lvl 5. Always heals 10% health per circle, other scaling is irrelevant. Use the attribute to set it to ignore enemies when needed.
-Safety zone, really good when placed on a tank or squishy.
-Guardian Saint, capable of reducing dmg taken by the target by 25% for a minute. Provided you don’t get hit. It’s useful at least, but otherwise in a undesirable circle.
Priests:
-Aspersion is great to keep the sturdier chars alive. And the tank should wear plate anyway. Nigh useless on cloth users though.
-Monstrance is very good for added dex for high dex builds, but it competes for better buffs tbh.
-Revive can be attributed to last 2 minutes and it affects the entire party. Meaning any player can die once every 2 minutes and still survive. Excellent back up skill.
-Mass heal is a instant aoe heal, very reliable and scales excellently with skill lvl. Almost a must have for a healer.
-Resurrection to res, keep in mind any damage will interrupt it. Priests shouldn’t need it often though.
-Stone skin, need to test it still. But huge amount of block helps anyone using a shield.
Bokor:
-Mackangal can keep someone alive for 20s, ideally combined with revive or something else to prevent the downside.
Dievdirby:
Not actually that useful for pugs, you’d have to sit back just to prepare totems every time. The stationary aspect really hinders it. Lower cd, invul and silence are in principle very useful though. Casting times are also undesirable…
Pardoner:
-Indulgentia, removes conditions. Can be decent.
-Increase magic def, standard dmg reduction for some spells.
Paladin:
-Resist Elements, incredible defensive buff that can outright nullify damage.
-Barrier, circle of squishy protection. It completely negates melee mobs and grants magic def, really good option,
Druid:
-Sterea Trofh, 10s invuln when used on grass/Chortasmata.
-Transform/shape shifting, a entire bag of tricks tbh. Can even bring additional heal spells due to it.
Oracle:
-Counter Spell, should prevent dmg from magic circles. Niche but useful.
-Prophecy, condition prevention. It’s decent.
-Forecast not sure if this is a team spell, but it should help you avoid damage.
Any other builds will have to compensate with offensive buffs and or damage to kill enemies fast. It might not be what teammates want out of you though. As typically most characters can bring plenty of dps/buffs/utility but rarely outright healing or dmg prevention.