Will replacing the hydraulic unit fix the problem?

I have a 1994 Ford Explorer and its brakes have been working bad lately. It hits the floor most of the time when I break. I took it to the shop to see what was its problem and they said that air got in the brake fluid in the master cylinder and hydraulic unit, and wanted me to replace those two parts along with all the hoses and callipers, which would cost me over $2000 including labor. My dad told me he's going to by a used hydraulic unit or master cylinder or both (things are cheaper where he lives) and replace it for me. He didn't really specify which one he's going to buy and I don't remember telling him which one, but it's one of the two or both. If I replace one of those or both without replacing the hoses and callipers, will it fix the problem? Or is absolutely necessary to replace absolutely everything?