Ok so while I have been a fan of Warmachine and the RPG side for a while now I haven’t really had a chance to play or run a campaign since 2015. I used to collect all the faction books just for the lore and the stories and all that fun stuff. I have gotten all the current books for the Requiem and I am super excited to dive back into it. How important is the current lore to follow? I not had the funds or time to get into MkIV (I was in the military and single when I played more, now I am a full time teacher, married and have two kids, so my free time is rarer now), Is there some way to get access to the lore of the Eternal Empire of Dusk? My group has decided that one of the characters is going to be a soulless who has the ultimate goal of making himself a soul and to be connected to House Kallyss. Darker than I expected but I am really excited to dive into the nature of souls in this game. What are anyone’s thoughts? Is there any material I should really look into?
Requiem takes place about 5 years before the Mk4 storyline and all the lore you need to cover the gap between Mk3 and Requiem is in Requiem itself.
Soulless and Dusk lore specifically is covered in the Borderlands & Beyond expansion for Requiem.
Oh I thought it was concurrent but Requiem is before Mk4? That i didn’t know. Does Borderlands & Beyond have Dusk equipment too?
Requiem is set in roughly 617 AR. That is 5 years after the infernal war but 5 years before the Orgoth invasion where MK IV starts.
As SaltTitan mentioned, you should download the Warmachine App and subscribe to get access to all the lore. If you were around way back when, then essentially think of the app subscription as getting a No Quarter each month. 
It’s definitely worth it!
I’m not sure how much equipment it has for Dusk. I think there’s a bit, but Requiem came out a bit before Mk4 was announced so I’m not sure how much they included
If you prefer, you could certainly set your campaign earlier than the default, or even later. You could run a campaign during Vinter Raelthorne’s reign, for example. Pick a period and have fun with it.
I am actually kind of excited for the story arc some of them want to try. Like I said one of the characters is trying to make himself a soul, since he is a soulless. I am giving him a bit more agency than most soulless have (I remember soulless having less individual agency and needed to be overseen and sometimes told what to do but I might be remembering that wrong or it might have changed) but it could lead to some fascinating places. I only recently found out more about Kallyss’s history, that ALL Iosans (expect soulless) in Ios either died or become Soulless when the last gods died (Did not see that happening with Goreshade’s resurrection) but it gives them a chance to dive into Cryx and Cyriss stuff, even possible some Skorne and necromancy stuff with the purpose of finding out what a “soul” is and what its worth (getting some Full Metal Alchemist vibes). It’s darker than storylines I have done before but could be really fun. So I’m trying to catch up to all the lore I missed since (I think) MKII when I played more.
borderlands and beyond has some Iosan equipment. And even soulless as a race. It focuses on Ios and Rhul. It’ll help with the elven stuff. The other lore you need is in the base Requiem book. The other source books are great too. My group has been playing from level 1 and are now 16