Generally you'd like to get any armor that Favors your class, in your case PGL or MRD in the Favors section, or anything that Favors Disciples of War.

As far as melee stats, STR boosts attack, DEX boosts accuracy/evasion, and VIT boosts Defense and HP. It's really up to you what you think would be more important for you in gear. As long as you get gear that is at your level or below, you should have no problems. Definitely stay away from buying gear that is above your level if you can, as you get reduced defense and no other stats.

You can share gear between your classes as long as they meet the equip requirements.