You can use the Lincoln WeldPac 100 for sheetmetal, but its highly recommended to use the argon shielding gas, especially for thin sheet metal like 18 gauge or thinner. I dont know if the one you have comes with the hardware to use the shielding gas, the one I bought only came with flux core wire, the hardware/hoses for sheilding gas had to be purchased separately. Anyway, I used the Weldpac 100 with flux core wire for welding in my floor pans, and it worked well but found myself blowing some holes in the metal here and there (the argon gas option helps you weld "cooler" and hence thinner sheetmetal). For body panels you really should use the shielding gas.
Good luck!