Regarding the 1120, you can depower the kite fine with the rear-most attachment points, but you can get more depower by moving the rear attachment points forward up to 1/3 the distance from the TE. This also increases turning speed. This will get you plenty of depower. At 58kgs (135lbs), you may want to go with the 840, instead, unless you plan to ride in 8-10mph winds.
If you're looking for any tips, you can check here for some side-launch illustrations and instructions to do it safely:
With an 1120, the downwind launch may be too risky for you if you're just starting out with a large kite like the 1120. There's more description on the rear attachment points, too.