This sounds like my idea for an adeptus mechanus army.
The horrors would have been Stormtroopers with insane looking lasguns, and some kind of energy field. and the dude with the bolt would have been a multimelta servitor. The other ideas seem solid. I suggest crazy looking bikers for fiends (or maybe some riders on some kind of spider thing, idk)
My Soulgrinder would have been a Huntsman Spider Tank (google it if you have no idea what it is) Sadly though, once I had a good look at the daemon codex I thought if I was going to play daemons, they would be daemons.
Still, I wish you luck!