If you need a new roof, give metal roofs serious consideration. A metal roof might cost more initially but it will likely last at least 50 years. An asphalt shingle roof will usually last 15 - 20 years. In addition, a metal roof will not catch fire from embers from a nearby home that is on fire.
If you decide to go with a asphalt shingle roof, we recommend the removal of any existing shingles to start with a clean slate. Removing the shingles will allow inspection of the sheathing including any repairs that might be needed. Also, each layer of shingles increases the load on the roof. It might save a few dollars to not remove the existing roof but in the long run it could cause future problems.
I am a full-time home inspector servicing all of South Carolina.