Whether you need a heavier low string comes down to how the low D feels to you. A heavier low string brings back some of the firmness lost by tuning down, so it's worth considering if the string feels too loose. It isn't required.
If you like your A, D and G strings and just want a firmer low D, S.I.T. Single Strings let you replace only that string. For a full set, Power Wound Nickel is a versatile nickel-plated roundwound, and Power Steel Stainless adds brightness, attack and note definition.
Drop D doesn't require a specific construction. Roundwounds and flatwounds both work. If you keep the same gauge, you usually won't need a setup. A noticeably heavier string can affect nut-slot fit and intonation, so check how the bass plays afterward.