Many items require vendor-bought materials. If you ever see white quality items needed for crafting, they are available on crew skill vendors which are on practically every planet in the game. You can also do gathering missions for them if you feel like saving a small amount of money, or would like some without heading to an outpost.
Actually, these missions can be quite lucrative if you go after the rich missions for them.
For instance, as a Scavenger at 400, I am able to send my companions on a Rich: Thermoplast Flux mission. For ~2000 credits I can get back something like 32-48 Fluxes. They cost 400 credits each. So I save at least 14000 credits each and every time! It's hard to beat that.
I think scavenging (and hence armormech, armstech and cybertech) has the only max-level rich mission for that kind of thing, but it's certainly not something you should ignore. I've certainly saved hundreds of thousands of credits that way already.
4. I'm didn't cover this directly but I was assuming this was a given. I'll edit this in at some point. All missions can crit, including lockboxes.
Yes, my point was more that the varying mission skills give varying results on their lockboxes. It may be just luck on my part, but Treasure Hunting lockboxes tend to give armor far more frequently than the Slicing Lockboxes. That may be because Treasure Hunting simply has more variety of Lockbox missions than Slicing, making it easier to go after the Abundant Lockbox missions.