BG233217171 It's the built in ceramic antenna; It won't require any antenna wires sticking out.

BG233217171 ELRS utilizes the standard Crossfire CRSF serial protocol for both RX and TX. Depending on the flight controller and firmware, e.g. Betaflight, iNav, KISS etc. CRSF should be easily set up through any free (uninverted) UART with the configuration software, or use the pre-assigned UART specified by the FC connection diagram or manual. Check the ExpressLRS github project page for more information, link is in the item description.

BG233217171 The difference between the models is the usable frequency bands. Theoretically, the ~900 MHz transmitter has a longer range and is less affected by obstructions, but has a narrower set of radio channels to use than the 2.4GHz band. 915MHz bands are used in the US and Australia, but the transmitter can also be set for use in the EU 868 MHz bands, or buy that option for Europe and other countries that use that band. Realistically, the 2.4GHz model also has a maximum range in the tens of kilometers and has more legally usable channels. Rare few video transmitters can operate as far either. The 2.4GHz version has a higher maximum control signal refresh rate, which can improve the responsiveness and feel of your RC models. It also requires smaller antennas and the EP2 receiver even has a built in ceramic antenna, weighing only at 0.44g, while a 900 MHz receiver requires an external T-antenna that is larger than that of the EP1 receiver. Both modules should work with any OpenTX transmitter with a JR module bay. However, the 915 MHz version doesn't appear to come with a module housing, so you'd have to supply your own JR module project box. If your concern isn't maximum range, then the 2.4GHz module and tiny receivers are likely the most versatile option.

BG233217171 Output power is adjustable from 10-250 mW (10-24 dBm.) Based on user test results on the ExpressLRS github project page, with an unobstructed view, even 100 mW should work well over 30 km, limited mostly by video link.

BG233217171 The plugin TX module Happymodel ES24TX (SKU 1835581) should fit the Q7X, X9D and other controllers with a JR-module bay. It requires the ELRS LUA script to be installed on the SD card to be able to change settings. OpenTX needs to be at least version 2.3.11 to be able to upgrade the module's ELRS firmware. Note that the PP RX's firmware is different than that of the EP1 and EP2, so don't mix and match between those models.