I've seen +12 Dexterity on at least one amulet or earring (pretty sure it was an amulet), and my current earrings have +11.
On level 42 my belt has +6% Crit; both rings are +7%. So far I don't recall seeing higher for either.
I don't see a section on the overall range of buffs available, but Lancet's
Dragon Knight Saga FAQ/Guide lists the buffs for various unique equipment, and gives the ranges if they are random, etc.
