I strongly recommend you get in touch with a local reenacting group that does what you're interested in. It is much more likely that they will be able to guide you through this process and prevent you from getting ripped off/buying sub-par kit.

This depends on your status as a soldier. Many veterans - particularly from prestigious regiments - kept their uniforms when Napoleon was exiled in 1814, and simply put them back on when the campaign begun. Due to the haste of the 100 days campaign and the need to field manpower quickly, however, many new recruits were given as little as a greatcoat, shako, and musket (with ammunition and accessories, of course) and wore their civilian clothes underneath the coat. And then there were the monarchist soldiers, who would have used the uniforms and equipment they were already issued with, simply swapping their royal cockades for the tricolor of the republic.

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So in short - it depends! You'd need to research which regiment and battalion you were depicting to get a definitive answer.

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Source: Napoleon's Wars: An International History, 1803-1815, C. Esdaile