Can't test now but I'm 95% sure that Paladin/Ranger caster level is either character level -4 (as it should be) or just always character level (it was this way in IWD1 IIRC). 1/2 caster level is not correct according to any rule and I've never even heard of that in D&D.
Most spells don't even care about caster level. Defensive Harmony? Animal Rage? Insect Plague? Literally couldn't care less about a low CL.
You can lob fireballs and shit on top of Deep gnome monk all day.
OK, so? Doesn't make them good, it makes your aim bad. btw you can get evasion with 1 monk level dip and the paladin will have better saves so a paladin wins anyway.
They eventualy get 1-20 dmg
Fist damage is INSANELY trash. That's 10.5 damage per hit. Holy Avenger will deal 1d8+5 +2d6 vs. evil = 8-21 damage = 15 damage per hit. The Paladin will also have like +10 AB (+5 from weapon +5 from better BAB progression) so they'll hit at least 2x as often with their attacks, maybe 3x as often. Monks are a joke class.
have bigger AC than Paladin.
Yeah, it's a 1 level dip max.
Barbarian will have 7 DR vs melee and 14 DR vs missile with abishai armor.
Anyone can wear Abishai armor if they don't want unarmored monk AC bonus. Barbarian has 3 DR at level 18 which is... incredibly meh.