If you buy second hand or new seperate components it good to know each manufacturer has its own type of connectors. They look comparable but convince yourself the servo connectors fit into the receiver. The Graupner 4-channel activates only 2 servo's. A radio set's airborn components. Note that most new radios are sold as sets that include all of the items on the list.
Batteries are often the most neglected items of radio control.
But I advise you to see them as the heart of it all. The power consumption of RC systems is rather high for batteries. So the use of dry alkaline batteries must be discouraged. They are relatively expensive and not very friendly for the environment. Examples of battery packs for the airborn receiver.