Mk.6 power armour is the easiest sort to convert; the vambraces, hands, feet, and pelvic area are just like those on Mk.7 armour, the greaves are a solid peice instead of the separate kneepad on Mk.7 armour, the breastplate is flat with a few cables on it, and the shoulder pads are rimless. Plenty of the normal
40k Space Marine kits have Mk.6 bits in them, and the rest are the product of a moment's work with greenstuff and a knife over the Mk.7 components (except the heads (a bit more work, you may want to find beakie heads from bits-sellers) and the breastplate (shaving the entire eagle off may be annoying enough to go source the flatter breastplates that exist in some kits). Pictures at
http://fromthewarp.blogspot.com/2012/01/mk6-space-marine-armour-conversion.html from someone else (not me, I like Mk.4 too much) who's done it.
As for the jump packs Forge World sells Mk.2 and Mk.5 armour with jump packs with the twin-thruster design on the Chaos Raptors, but the Dark Furies' jump packs look like a variant on the single-thruster design on the Mk.4 Destroyer models. If you can find them the Sanguinary Guard jump packs (the bits in between the wings, you can just leave the wings off) are perfect, but everyone who's playing 30k has had the exact same idea so they may be hard to find. You could try taking the Chaos Raptor jump packs (with the spikes shaved off, of course) and cutting them in half to get a single thruster (and end up with something that looks kind of like the Usarax jump packs), maybe? They'd be a bit small but perhaps they're low-profile stealth jump packs?
As for Decapitation Strike it looks to me like it's Orbital Assault lite; you don't get as many Drop Pods, but you're allowed to deploy your Recon squads with nuncio-voxes by Infiltrate. The issue I have with it is that all the units you're putting in Drop Pods could just be Infiltrating via By Wing and Talon, so you're giving up a more useful Rite to take Deathstorm pods as Elites instead of Heavy Support (which isn't a very crowded slot for the Raven Guard anyway) and keep your infantry off the table for longer. I'd say Drop Assault Vanguard (which takes away your Recon squads but is much better for the Assault squads) and Recon Company (which gives your infantry Shrouded on turn one, and you don't care about the downsides because your Heavy Support choices are Deathstorm pods deploying in Reserves anyway) do what you want to do better.
(If it were my army I think I'd do Recon Company; it makes you take more Recon squads, but it gives you more bonuses, you don't get stuck with your Troops assault units on the table and the Dark Furies stuck far off the way you do in Drop Assault Vanguard, and none of the Rites really do much for Dark Furies anyway.)
