Role Level unlocks the right to buy a specific weapon or vehicle, while Cash is the currency you actually spend to purchase it. You need both, a high level with no Cash still locks you out of good gear, and Cash with a low level has nothing worth buying yet.

You don't need to level all six roles, since your overall WARDOG Level is just the sum of the six, but focusing on one role gets you its specific gear unlocks faster, while spreading play across all six raises your overall level more evenly.

No, Cash on its own doesn't unlock gear. Higher-tier weapons and vehicles are locked behind Role Level first, and Cash only becomes useful once your level has made that item purchasable.