Just started doing this so their repair skill gets better. Unfortunately, Lucky Harith just bought it outside of Temple Of The Union, after I sided with Leroy Walker and the Slavers. We were walking back and came across his corpse and wounded guard and brahmin. A enclave corpse followed. I doubt someone replaces him in the game? Still have the other three, and guns aren't my concern. Mainting leather armor is my highest priority anyways. Lether Rebel wears down too quickly.

Once they're gone, they're gone, I'm afraid. No replacements.

If you're playing on PC then you have a couple of options. You can resurrect them, although they will resurrect in their 'uninvested' form and I don't think you can re-invest in them. You can also modify the health of the living merchants to make them tougher.

On console, your only real option is to try to stay away from the merchants as much as possible. The game keeps track of where the merchants are on the map, but it only actually renders them if you are within a certain distance of them. If they are not rendered, they cannot be attacked. Basically, if you see a merchant caravan and you don't want to trade, use the 'wait' function to send them on their way.

Here are some console tips for caravans, I'm on 360, but assume this will work for PS3 as well.

1. Now in the early levels, it's tough to get armor repaired decently. Even though this is the case, as fast as I can get a decent sneak skill and a stealth boy, I reverse-pickpocket the caravans one by one. That is, I get two decently repaired armors... you know, like Raider Badlands, etc, whatever is common out there in the world, at your current level.
Then I go to Rivet City and wait until night. I SAVE my game. Hard save. I wait in two hour increments until a caravan shows up. If it's the Doc, I tend to pass him by, as I usually buy you-know-what off him early in the game, though I don't actually use it until late in the game. He tends to die right away for me.

I go into sneak mode, equip the stealth boy, (do this in your underwear in the early levels), wait until I am hidden, then try to RPP one of the two. I reload if I am not successful. After I successfully give one person armor and headgear, I make ANOTHER save. And then do the guard.
As I advance in levels, (these days I tend to use MQ as a vehicle for advancing levels of enemies, so I can just be in my RP.) I find better armor.
What I'm saying is that eventually all my caravans are equipped in Enclave Power Armor, wielding gatling guns and laser weapons: I gave them ammo too.
And at later levels when you can easily pickpocket them in broad daylight, this instance is really the only time I use the Chinese Stealth Suit, you will find all the old armor you gave them, and can take it back from them. Sad I know, about the CSS. I don't like being OP.

2. My caravans are absolutely safe in every game at only two locations: Rivet City and Paradise Falls. Your mileage may vary, but my caravans never get attacked at these two locations, every other stop they have, it's a blood bath. So I go to these two locations to trade only. THIS ONLY WORKS WHEN YOU FAST TRAVEL. If you infiltrate like I do, that is, no fast travel, then it will be a bloodbath regardless. Megaton is generally safe in the early levels, but, for example, I just killed a Deathclaw outside Megaton's gate. So I have feeling that trading out there is not good anymore. The body is too heavy, I can't drag it. So it will respawn right there. Hopefully it respawns at night, and moves on after killing only Weld. Who seems to come back to life for me every three days.

All this being said, I don't go this route anymore. If they die, they die. But this is the way I figured out to keep them all alive forever. Especially Wolfgang and his awesome repair skill.

I tend to use the caravans as my main source for income beside running around and killing everything (i buy some things i use there for continuing quests) and its a pain when they die i cry my ass inside out. Your info really helped, but i have never succesfully done reverse pickpocket. If u fail can u try again in a day or so?

Just remember to save your game before attempting. Then if you fail, you can reload. You have a chance of failing with each item you place. After I successfully get one person covered, I make another save and go for the second. You will always fail if you are overencumbered. So effectively, I can only cover one team at a time, which means a little more exploring for four stealthboys, things I find rarely in the first several levels, if at all.

Is it possible to drop things in front of them and they pick it up? in all my years of playing fallout i have noticed they pick up guns if they see, but armor or ammo?

Only if it's better than what they are carrying, but I don't know their packages, I only speak from watching the chaos unfold. I find that raiders will pick up anything and everything that is better. I have been killed before by five raiders, all wearing Enclave power armor. I used to RPP power armor on raiders so I could create big battles with the Enclave. I would rpp when they were alive, so that when they died and respawned, they would respawn in power armor.

I find it better just to give them what I want to give them. And don't forget the ammo. As like Protector Casdin, he can carry 2000 enclave armor suits and crap and never be overencumbered, so are the merchants.