868 MHz is right
For dBi and directionality, it depends on terrain and goals. In MeshCore you can ask the signal to follow specific router paths, and so you may or may not want an omnidirectional antenna depending on how you want the signal to travel.
Most common antennas that you will find use wither SMA or N connectors. For outdoor use, N connectors are allegedly better in terms of water resistance. If you are making your own enclosure, you can find both SMA-IPEX and N-IPEX connectors with an O-ring which prevent water from entering the hole through which the connector threads come out. The inside of the enclosure is protected equally well for SMA and N, it is just the connector portion which remains exposed, so over months/years the connector may corrode and N-type may last longer. The Heltec T114 is probably shipped out with an SMA-IPEX cable with no O-ring.
I use this one: N-type connector, 8 dBi, omidirectional:
https://nl.aliexpress.com/item/1005007463706065.html
