AD/DNS/DHCP doesn’t require much space, so you can go with 2 x 240GB/480GB SSD drives in RAID1. RAID0 shouldn’t be used in any case. RAID5/RAID6 are not cost effective here, because of 3 or 4 drives. More about RAID levels here - www.starwindsoftware.com/blog/raid-today
Don’t use HDD as it makes no sense with such capacity.
Don’t forget that according to best practices you need the second AD.