(...)one group would be escorting 1 wagon(...)
Players or NPCs? It's easy enough with players, there are no additional requirements. But with NPCs, how do you handle the AI? Should the caravan stop or continue during a raid? Do the guards keep locked to the caravan or do they pursue the bandits?
Because if they stay around the caravan, it's easy enough to pick them out from a distance, one by one. If they follow the bandits, you send in one bait, lure them off and then raid the helpless wagons.
the AI depends on how much you spend
if you buy the best geared (call them veterans or something) then they'll stay withing 10m of the caravan. maybe these come standard with better gear but still worse than player crafted.
if you buy the second best they may wonder a little further, say 15-20m away.
the new mercs are easily baited so they'll go even further!
maybe you could buy.. er rent some new mercs and they gain exp if they dont die!
another option could be you outfit them with gear or they use their own (which is worst to equal to bad player created gear). IE if a player has white tier gear as a base then they'll have a mix of gray and white. I only offer this because it's a player economy and crafters need all the means necessary to sell/trade gear.
I'll note that these AI mercs shouldn't drop gear as it could be easily exploited/farmed (if it's their base gear, player crafted gear can be acquisition!)