There is a choice of solid, semi-solid and engineered wood.
Yes, if the Solid floor is laid across the existing floorboards it can be nailed in directly; but if the floor is laid in the same direction as the floorboards then it is important to lay a 6mm ply before- hand.
It can be done but it is wise to get expert advice, as a professional will use a hygrometer to check moisture/damp levels before proceeding. Using Solid plank floors on concrete can be difficult so it is better to consider an alternative like Herringbone blocks for example, as they can be glued down onto concrete.
Generally, a levelling compound is used to level concrete floors.
To determine how level a floor is, it is best to lay a long straight edge on the surface. A few millimetres difference up or down in height on an all-over surface area of sub flooring is quite normal.
An Engineered or floating floor is mainly suitable for this type of fitting.
Yes it can, providing that an appropriate underlay is placed first. A hard board may sometimes be used if the boards are laid in the same direction as the floor boards for extra stability.