Measure before you buy or process anything, because you have at least three different problems in that list and they have different fixes.
Record thirty seconds of silence — nobody in the room, everything running as it normally does — and look at it. Then repeat with the computer off, then with the fridge unplugged for a minute. Three files, and comparing them tells you which source is actually dominating rather than which one you notice.
Most people are surprised. The fan is usually not the biggest contributor; the low rumble from traffic and appliances is, and it is inaudible as a sound while being very present in the file.
Then fix in this order, because it is cheapest first and each step makes the next less necessary:
Move the microphone, not the noise. Every doubling of distance from a noise source drops it substantially. Turning ninety degrees so the microphone's dead side faces the computer often does more than anything you can buy — a cardioid pattern rejects the rear strongly, and most people have the machine directly behind them without thinking about it.
Get closer to the source. Closer to the microphone means more signal for the same noise, which is the whole ratio. Half the distance is a large improvement, and it costs nothing.
A high-pass filter removes rumble that is below anything your voice or guitar produces. This is the one processing step that is not a compromise.