There is nothing functional that's only available for real cash - there are fancier skins but they do nothing you can't get for free in-game grinding. If you want a particular loadout, you can buy just that card, either with real money (booo) or in-game currency (hurray!), you don't have to gamble.
Basically, I would treat the random packs as 'freebies' when you get them in the game, and then use in-game currency to buy mercs or occasionally specific loadouts.
I'd say use the free mercs on rotation (I think it's to change every 2 weeks?) and only 'buy' a merc if it's off rotation and you really miss it. If this holds my attention I may buy the starter pack (£22.99, 5 mercs + 50k in-game credits) purely as a way to pay the devs for the game, same as I occasionally buy a pack of stash tabs in Path of Exile whenever I rack up another 50 hours.