Do I need a moisture barrier outside the garage wall insulation

Increasing your wall thickness to facilitate 6in insulation is a fine idea, can't hurt. A vapor barrier of 4 mil plastic with taped seams should always be installed between the insulation and drywall. this will stop moisture from the outside from penetrating the back of the drywall and conversely, humidity from inside collecting in the insulation. T111 siding is notorious for not being a tight exterior sheathing, especially when installed without a backer such as OSB or plywood covered with a layer of Tyvec. The interior of the wall will vent itself and ambient moisture in the wall will not be a problem in most cases, as long as there is not any water infiltration into the cavity, you will be fine.

Any high humidity area like a bathroom should have a good vapor barrier under the sheetrock. This barrier should be taped at the seams and ceiling forming sort of an air tight cell. Usually a kraft backed insulation is not a very good vapor barrier and has lots of gaps. To be on the safe side and avoid any condensation between the 6 mil and kraft, you could simply remove the kraft backing or cut several slices in it top to bottom. Usually it will not cause a problem unless you live in a very wet humid climate or the plastic is not sealed properly. The 6 mil is a much better material for your purpose than kraft backed.