This could be a deciding factor for the instrumentation. Acoustic instruments have the greatest flexibility. If your ceremony is outdoors, special consideration will have to be made. Some keyboards can be powered with a rechargeable battery or generator that can be set up near the ceremonial site. This will also power an amplifier in the event that your chosen vocalist needs a microphone.
Some singers are able to work well a capella and unamplified.
Once you decide which sound you like, you can choose the number of musicians to create the music and desired mood. The same musicians could also be employed to play during cocktails, whether in the same location or another location. The type of music to choose from has unlimited possibilites. The important element to keep in mind is that it has meaning for you both. Some couples are partial to recorded music that may have special significance, and it's quite acceptable to have a DJ with appropriate sound equipment program the choice of music.