Turrets have a collision sphere which checks for overlapping actors, puts their health into an array and fetches the one with the lowest value. The retrieved object is then set as target for the turret for as long as it’s alive, is in range or something with lower health comes along…
Apart from that there are some more elements when it comes to target selection. The project is attached if you want to take it apart, it’s just 3 blueprints, really. Some stuff in the construction scripts, some widgets and it all spawns in the level blueprint.
Link to the project (20MB): https://drive.google.com/open?id=1dmzXQAI3dQ9VSHsZp8J_il6Lx_d8YXPz
Vid: https://i.gyazo.com/beee742e871e5d4346310b688a22e54d.mp4
Hope it helps.