How difficult is it to replace drywall and siding yourself?

We have a separate garage that experienced some water damage due to a lack of drainage in our backyard. We've since fixed the drainage problem, but now we need to replace some of the exterior siding (not vinyl) and interior drywall (just the lower half of the wall). Do you think this could be a do-it-yourself project, or should we hire someone? I am a little concerned the damage could be more extensive than we realize. Thanks.