Y'all are crazy... but I love this board anyways.
Stormtrenchers:
Use the shoota's from GW Orks as they have drum feeds and Plastic Regulars. Jumpacks can be Wailing Banshee packs, Hasslefree Jet Packs or GW Jump Packs. *shrug*
Sterlings:
I got a bunch of the Old Imperial SF trooper Captains for mine. An alternate model *could* be a handful of Space Marines with non-helmet heads and the aforementioned Ork Shootas. They would work well, now I think of it.
Black Berets:
The SMG and Chainblade Imperial SF Sergeants. There are a number of possible alternate models out there - some of which could be the current SAS figs fro mvarious Manufactuers with a bit of modding and or weapon swaps. Just a thought. If you are good with the Greenstuff, snag some of the Catachan plastics, mod the guns (hell, monkeying with Bolters would work) greenstuff Berets and bodyarmor and there you go.
That is... if you agood with the GS.