It's only random if you buy the cases.
Click store -> loadouts and you can choose a specific one.
They are fairly pricey (10-17k ingame credits I think, I've not shopped around much) but if you know which one you want for a merc you are enjoying you can just go in and get it.
They are all 'Bronze' level, which means choice of weapon, 3 mods. The higher tiers (silver, gold, cobalt) just give the exact same bonuses and weapons with a different skin.
Ahh, I see. Don't like the idea of buying loadouts before trying them but hey ho