1985 BMW 325E fron brakes

I just bought a bmw 325 E for 300 bucks...GREAT DEAL! the only problem is the brakes....sometimes the brakes work fine, then sometimes the brake pedal will go all the way down to the floor with no pressure and do nothing to stop the car...ive been told master cylinder, brake boosters, brake lines, or wheel cylinder...i have no idea what to look for...