Whoever suggested using 2mm steel was probably joking (I hope)!
Fiat, used afaik steel approx 0.7mm for outer body panels, slightly thicker for underfloor strengthening sections (top hat/crossrails/outriggers) and thicker still for main chassis rails.
To find out what to use in a particular area, measure a rust free area with a micrometer. For front wings/fenders, clean up a wheelarch lip and measure this.
Given that steel suppliers are likely to only stock a limited range of sheet thicknesses, you'll probably have to do as 'Ramona' suggests and use the thinnest available, at least for repairs to outer bodywork.
There's little to be gained from using thicker steel - harder to cut, much harder to shape and easier to butt weld metal of equal thickness.
Sheet metal is measured in SWG (standard wire gauge), the bigger the number, the thinner the sheet (or wire). e.g.
16 SWG = 0.064 ins = 1.626mm
18SWG = 0.048 ins = 1.219mm
20SWG = 0.036 ins = 0.914mm
22SWG = 0.028 ins = 0.711mm
24SWG = 0.022 ins = 0.559mm
26SWG = 0.018 ins = 0.457mm
28SWG = 0.0148 ins= 0.376mm
In the US, measurements are in US, SWG, each numerical size above is very slightly different from the SWG size (by a few thousands on an inch).
For outer body panel repair, I've used 24SWG if I can get it, otherwise 22SWG - anything thicker is too hard to shape when making repair sections.
You may find that stockists only sell sheet in metric measurements - if you can't get 0.7mm, then 0.5mm would be a better choice that 1.0mm. For underfloor reinforcement rails/box-sections, measure what is already there and pick the nearest available metric size. If you only need small pieces of sheet metal e.g. to repair holes, you could use old body panels as a source of suitable thickness steel sheet, others have used old filing cabinets/lockers etc as a source of thinner material.
Bear in mind limitations of tools/equipment available to you - especially cutting equipment e.g. Sheetmetal Tinsnips will struggle with cutting 1.00mm sheet!
Have fun
AL.
