I'm just curious about this- if the courier was robbed shot and buried how does he still have all these weapons armour and other items? He's a travelling courier so he wouldn't bury items around the wasteland in caches, and if he saw Benny and the Great Khans coming, he wouldn't hide really good equipment. If he still had it when he was captured why wasn't it stolen?

I'm over anolysing this I know but is there any justification in game for him having these items?

Well, I assume burying it in a cache beforehand was EXACTLY what he did. The way I think of it, he probably buried them in case of emergencies, not at all unlike Dean Domino did with his stash. I mean, you never know when you might need extra equipment. As a courier he travels a lot, and the roads are sometimes dangerous, so I don't think it would be out of the question for him to bury something like that. As for why he didn't have it with him when the Khans came/why the Khans didn't steal it, well from the looks of the opening cutscene they attacked at night, a surprise attack more than likely, so he didn't see them coming. Hence, he did not have time to get the gear in the stash to fight them off, or the cache simply was not in the area.

That's just my opinion. More than likely, they didn't even think of a backstory when they made the DLC.