Mercenaries 2: World in Flames Hints

To unlock the extra costumes for each of the characters you have to beat Fiona's lvl 3 challenges: Gun Emplacements, Pistol, Grenade Launcher, and Sniper. Each character has 5 costumes total including the default.

At any time during the game when you have a vehicle that needs repairing or more ammo, you can get both for free by entering any allied HQ or outpost, you don't need to buy anything or even take a contract, just leave. Once you leave, your vehicle is fully repaired and rearmed waiting for you outside.