Since you already have the factory disc brakes I'd say use that.
Those aftermarket kits from eci and scarebird are meant to improve stopping versus OEM 9 X3 slant 6 drums at best.
You dont need to change the lower arms but you will ruin the lower joint boots in seperating. Replacement joint boots are hard to find.
Now I have questions for you... this power brake setup ? Is the pedal low or mushy ? You should have a high and hard pedal even if no power assist and using 2 feet to stop it today.
