If I were doing it, 1/8" would be plenty. Actually the steel that I use for body panel replacement is 16 GA. It is about twice as thick as standard body tin.
Cut out the bad parts and replace with your choice of steel, the thicker that you decide to use, the harder it will be to weld without burning thru on the original steel.
When I did mine, I re-drilled for the roll bar bolts, and made a plate that fit under the body to "sandwich" the repaired section and give it more support. The vertical part on mine was good and solid, so it made it easy to re-align the roll bar.