There must be something wrong with your character then. My Full INT level 280 Wizard is doing great.
Technically, with more INT your Heal and Cure will deal a lot more damage (and your Heal will heal more), since they both scale with INT. A lot of buffs scale with SPR, so if you wanna be a better healer, just go INT, if you wanna be a better buffer, go SPR.
Me, personally, I prefer the buffs over the heal. Heal doesn’t really matter once you get Mass Heal (it scales with both INT and SPR) in Priest2 (Just for the cooldown filler). In the end, it depends on what you wanna do.
Full/High INT is kinda for DPS Clerics, and it makes leveling easier. Two of the sacrifices you would need to make to be Full Support, is time and patience, since it will be hard for you to level up, unless you have a fixed party running.
Tl; dr: For Full Support build, I suggest Full SPR, Priest3<(Your Choices).
You don’t need to. You have either of these attributes available: Cloth/Plate Mastery (Choose one) which boosts up your SP+MDEF/HP+HP+PDEF.
Personally I think Cloth Mastery and SPR is useless, MP Potions may seem pricy when you first start your character, but later on it’s really cheap (Buy the ones that are lv. 10 Concentrated MP Potions from Alchemists). It’s better to level Plate Mastery and Full INT instead.
If you play your cards right, you don’t need CON, like, at all. I have played four characters without CON, and they’re doing fine.
The reason why I dislike spreading my points (even a little bit) on CON is because of how the status allocation points work in this game. For every additional class rank, you receive a bonus of 10% INT.
Also check the ‘Bonus Stat Points’ section here: http://wiki.tosbase.com/wiki/Stats
If you’re aiming for PvP though, that’s another story. Go Full CON. Full INT is for PvE.