So on the topic of a 'lost' chapter of Astartes. Here's what I am thinking...

They just need to come off as a 'lost' group of Imperials in general, with some Archeotech that is decidedly rare, but not greatly beyond Imperial norms (other than the density of Archeotech).

What they need to do is find a part of the Imperium that has been cut off by warp storms for at least a thousand years, with little to no records of what is beyond that 'wall' but where the warp storms aren't always the type that enter into realspace, or aren't ever the type that enter into realspace, thus enabling slower-than-light travel.

Then, what they have to do is have a group of ships show up, coming out of this area at _slower than light speeds_, saying they are from beyond the wall, with archeotech stasis pods on their various ships, saying they are refugees from a Chaos war on their planets, where Chaos triggered some sort of insanely destructive Xenos device that destroyed the planet(s). Luckily, it was a 'slow' destruction, and it uncovered a cache of Archeotech, allowing the elite of the planets to equip themselves and fly away back to Imperial space in a desperate bit for vengeance!

So a flotilla of ships, including a Battle Barge and several other Astartes ships, as well as a number of more Navy-style ships comes into some border system or something. These people (who havent been part of the Imperium for thousands of years) are somewhat confused by the current practices of The Imperium, but they are howling for vengeance for their dead families.

And they include a whole chapter of Astartes, lots of PDF and SDF (trained and equipped more like Auxilia to the Astartes Chapter, and thus at a higher standard of equipment), and several Knights (this apparently was also a lost Knight World) -- all 'local', from a 'destroyed' set of planets...

If the whole thing is a fleet equipped wayyyy beyond the 'standard' Imperial, and closer to the 'archeotech theoretical', they should be able to punch WAY above their weight class... without giving the enemy access to Culturetech. The fact that Imperial tech is often schizotech, with rare, occassional leaps to wayyyy higher tech than average, means that if someone made a concerted effort to equip a LARGE force with the best of the best, they would be able to do things that most Imperial forces couldn't do.