The highest caster level needed to create a magical item in the game is Caster Level: 18 (Invulnerable Armor, Scarab of Greater Protection).
Caster Levels don't stack, so a Cleric 11/Wizard 9 only has a Caster Level of 11.
A number of spellcasters can create magical items in the game including, but not limited to, wizards, sorcerers, druids and clerics. A wide variety of clerical, druid, and arcane spells are utilized and although there is a tremendous amount of overlap, no single caster can cover them all, even with the use of the UMD feat.
Using a Scroll to Craft should give you an effective Caster Level of 10, even if you're a higher Level, a lower Level, or a non-Caster Class using the 'Use Magic Device' skill.
We recommend spreading the Craft Wondrous Item and Craft Magical Arms and Armor feats among the various NPC spellcasters. You might waste a feat slot or two, but if you are going to craft weapons it's well worth it. As of v1.03, Warlocks can not take the Craft Magical Arms and Armor and Craft Wondrous item feats.