Every one of these runs Meshtastic, and most also run MeshCore. Pick by how you will use it, not by spec sheet.
UK buyers: always choose the 868 MHz (EU868) version. Cheap overseas listings often ship 915 MHz hardware, which you cannot legally transmit with in the UK. Get a proper tuned 868 MHz antenna too. It is the cheapest range upgrade there is.
Under about £30. Bare boards to tinker with.
About £30 to £80. Finished trackers and kits.
£80 and up. Standalone handhelds, solar repeaters.
Buy two. You need someone to talk to while you learn.
nRF52 boards (RAK4631, T-Echo, T1000-E) use very little power, which makes them best for batteries and solar. ESP32 boards (Heltec, T-Beam) add Wi-Fi but drain batteries faster.
Stock "stubby" antennas are often poor. A tuned 868 MHz antenna, mounted high and vertical, can add kilometres. Never power on a radio with no antenna fitted, as it can damage the radio.
Most boards flash from Chrome or Edge in a couple of minutes with the official web flashers. Pick Meshtastic or MeshCore, choose your board, set your region to EU868.