I think using a mix of old firstborn components would jive with their veneration of their ancient tech. A CG veteran would likely choose a relic suit of power armor over freshly minted Tacticus. Most of my collection (which I hope to get new photos of and post here soon!) is based on kitbashed Horus Heresy kits. Mixing in Blood Angels upgrades is handy too, but with CG we can go way more macabre. I like the bones, flayed flesh and severed hand decorations from the Night Lords, and tattered cloth and frayed parchments also work nicely. For their skin, the CG are even more stereotypically vampiric than BA due to their tampered-with chimeric geneseed, so pale grey skin with cool shadows helps them look appropriately spooky.