Be really safe and choose black or basic, somber colors. It will take some time and research to learn what types of prints and patterned fabrics were available during the period. Don't run out and spend money on patterned fabrics until you are sure they are period correct. Just because you saw it at a sutler or even on another re-enactors doesn't mean it is period correct. Always wash and dry your fabric before you cut out the dress.
Cotton can shrink, avoid making alterations by using pre-shrunk fabric.
Do not plan on adding any lace, braid, or trim . This is time consuming, and until you learn more about what types of trims were used, and where to find facsimiles, leave it off. You can always add trim later when you have more leisure. If you like, dress up your garment by adding a collar and a nice self-fabric belt. Forget about adding piping to the armscyes of the dress as instructed in most patterns.