It sounds like your basement may have a moisture problem, a common situation for many homeowners. Before starting a basement project, you need to fix that. Be sure surface drainage is adequate. Angle the soil away from the foundation, and check rain gutters and downspouts to be sure they’re channeling water away from the house. Other measures may be necessary to solve a moisture problem in the basement, and I would encourage you to work with a remodeling contractor to address any issues.
You should also test for radon, and if the levels are high, have a professional install radon mitigation measures. Then check your home’s heating and cooling equipment to be sure it’s sized to condition the air in your basement as well as the rest of the house. Now would be a good time to upgrade if not. Building codes will likely require at least one large window for an alternate route out of your basement in case of emergency.
Taking care of issues like these will help ensure you’ll have a wonderful new living space in your home. Professional remodeler Dean Herriges offers some excellent tips about basement remodeling in this video.