Well he's mostly correct because they are bought for daily comms so someone is grinding dailies to get them. 2 of the mats cannot be gathered by missions either.
I don't think the effort in gathering these mats yourself or alternatively the prices charged on GTN will make these items viable - my server is looking in the region of 400k on the GTN for the mats for 1 core, so buyers are looking at millions of credits to fully equip HK with these parts?
If it was me, and it is I guess as I'll get him, I'll stick with a companion equipped with 'free' columi/rakata pieces I've got spare or at worst, I'll make myself some custom parts, and rip a few BH mods out of my next drop to make him comparable to the items (so 3x18k rip costs per item).
You're forgetting about the Droid Armouring issue. Droid armourings aren't head, chest, legs, hands, feet and thus you'd be questing with a SEVERELY gimped companion if you did that.