The end of the game is when the guy is laughing at you, turns the lights out, you fall down a hole, and you have to mount a turret gun. So use your ammo, health packs and sentry guns at will. I had plenty at the end that I didn't use.
My suggested upgrades: - Assault Rifle - Shotgun - Health
With the rest of your $$, purchase ammo for those 2 guns mostly, and LARGE health packs. Occasionally, a sentry and life jacket.
The campaign on Elite I give a 4/10 difficulty, but with the mp achievements, I'd give this game a 6/10 difficulty.