You shouldnt need anything more then 1mm for that type of work...2 mm absolute max.

Also, if it is a normal repair then I would try going to a breakers and cut the same bit out if you can as would save a lot of shaping and the steel would be the right thickness.

Cordless angle grinder is a good asset for this type of work and use a 1mm cutting disc. An air saw is useful on the car and a plasma cutter is a nice luxury.