So I've been playing a bit of online multiplayer, getting some money, and basically buying out everything I could in the shop in order to force the gear (not weapons, their unlock method is explicit) to cycle and hopefully give me some new choices. I'm not sure if leveling up is the main factor that determines getting new gear, or if it just happens when the news broadcasts rotate maps.
The clothing shops get new gear every day. Your level will determine the availability of 2-3 star items in the shops.
Additionally, beating the single-player campaign will grant you two sets of 3-slot gear. Along the way, as you mentioned, several weapons will also be unlocked for purchase. The best resource I've found for these weapon unlocks is here. Anything without a "Single Player Scroll" entry is unlocked exclusively through multiplayer, and made available for purchase at the level indicated in the left column.