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Choose meaning a brand. You could run a adjustable or a fixed length lower for caster angle.
In bold I mentioned the rear axle side track bar Bracket. Besides roll center correction the axle is centered under the jeep. So no, you do not need a adjustable rear track bar.
A 12.5 will work but you might have issues having it mounted. Tire manufactures usually have a min wheel width and shops won't mount for liability reasons.