The good news is completing the game ought to have got you a lot of prestige points for the shop. Use those to buy Prestige crates. When you eventually start getting duplicates from the Prestige crates, you'll start getting store points to buy other stuff that's listed for points. (Items in the ingame store with a real-world currency cost must be bought with real money - the stuff available for points can be purchased free of charge thanks to the duplicate prestige crate item rewards)
Well, the daily challenges ingame will net you a possible 1050 prestige points every day (800 for completing the 3 solo challenges, 50 for the community challenge and 200 for the Task Force challenge) - and you'll get crates as an additional incentive for completing set amounts of (solo) objectives each week - all of which will eventually add to the store credit, allowing you to unlock everything that's not reserved to the DLC packs.