Hello Lukasz,
Your best bet is to use UWorld. You can get the instance of the world that any AActor exists in by calling their GetWorld() method. The class itself provides a bunch of world sampling methods, such as OverlapMulti. OverlapMulti will return a list of colliding AActors from multiple primitive types, including a sphere. Here is some example code:
TArray overlaps;
AActor* Origin = 0;
float Radius = 150.0f;
if (Origin->GetWorld()->OverlapMulti(
//output list
overlaps,
//origin location
Origin->GetActorLocation(),
//origin rotation
FQuat::Identity,
//collision channel
ECollisionChannel::ECC_Pawn,
//collision primitive
FCollisionShape::MakeSphere(Radius),
//collision parameters
FCollisionQueryParams()))
{
...
}