Body / sheet metal question ???

[BILL WZOREK]

I plan on buying a 4'X8' or 5'X12' sheet of metal to do body work AKA
Forming pieces as needed, If I understand correctly ( this may be the wrong terminology ) the steel can be made up of different content of tin / nickel / other alloys. I want a all around general type of material.
Some thing that is easy formable but yet easily weldable with my mig welder.
What Thickness should I get?
What compound mixture or % of each?
In general what should I ask for?