3 Class : Pathfinder, solider, and juggernaut.
With this classes, you can access to the majority of roles availables (capper, chaser, defense, harass, etc.) The only big lack is the sentinel, which is the only class with a sniper rifle (important for the defense)
Anyway, the price to buy a new class (with its base weapons and perks) was very low, it’s only the new item (weapons, belt, perk) which are expensive.
So with the free version of the game, you can quickly access to the majority of classes.
For Dirty Bomb, we can consider they will offer 5 mercs at the beginning, 1 for each class. So everybody will be able to fulfill one of the roles proposed.