Standard C5’s would work with A5c/KP at 2500-3000 feet, provided that there are no factors that dramatically reduce SNR (lots of trees or heavy interference). The answer here quickly turns into an SNR modeling exercise with consideration for your throughput expectations. Some users are happy with lower performance on longer links spanning several miles in quiet rural areas.
For the best performance, all clients should have similar Rx power at the AP, and it should be high enough to squelch out noise with AGC without dropping the weakest client. Better to have them all come in hot, and at the same level if possible.
The C5c has RP SMA connectors, so there many existing 5 GHz antennas that would work…parabolic, patch, etc. depending on gain and aesthetic requirements. I’d like to hear from the community what antennas they use for other connectorized clients (don’t be shy - you can use other manufacturer’s names).