You'll have to find guys who work for cheap that can do the job. Later, when you've got cash flowing in from the sale of fallow tree sap, you can start putting more skilled mercenaries onto your team. It's always important to remember, however, that your goal in hiring mercenaries is to get the right MIX of talents. You need an explosives expert to make and disarm bombs and traps in the field. You've got to concentrate on building a team.
Inventory management remains a vexing issue throughout the game.
You cannot buy weapons, ammunition, or other inventory items. You are limited to what your mercenaries bring with them and what you find on the island, on the bodies of dead enemies, buried in the ground, or stashed away in crates. Your mercenaries cannot carry an unlimited number of items, so you're going to have to make some tough decisions about what items you want to carry with you and what items you can leave behind. You've always got to keep an eye on your inventory because most items are going to be in much scarcer supply than you'd like. Think twice before tossing that last grenade because you might need it later and not have one.