If you go out and aggrivate a person, get them mad enough to come to your apartment and kick the door in, at which point you shoot them, you will likely be charged and convicted of murder because you 'baited' them into attacking you so you could shoot them.
It doesn't matter if that was your actual intent, by going out and starting somethign, and later finishing it with a firearm, that is how it will be treated.
Here is how you should handle the situation.
#1 MOVE
if option #1 is not financially feasable then #2 is
-Get a video camera, voice recorder, whatever. Whenever the guy is loud, start recording, and call the police. Write down the time of noise, time of call, time of arrival, time of departure. Contact the building owner and complain.
IF the guy quiets down when the cops are there, and then gets noisey again (say, in an attempt to intimidate you) begin recording and call 911 again. Contact property owner again.
NEVER bang on your floor in response. NEVER go down and confront him, and NEVER do dumb things to 'get even' like egging his car, letting air of of his tires, or hanging up posters.
This is part of the reason to be recording as well.
If you EVER have to use deadly force, you want it to be clear that YOU are the CHOIRBOY and HE is the HELLION.
Now, the ONLY time you should ever let him see the gun is if he successfully busts down your door, and which time you point the gun, scream STOP FREEZE DON'T MOVE GET DOWN or whatever. The police should already be on the way, as any banging on your door should have caused you to call them BEFORE grabbing your gun.
Now, of course, if in a roid rage he may not care, or think you are bluffing or whatever. If in the face of a firearm, he continues to advance into your home, aim for the center of his torso and shoot. Your goal is to shoot to stop the treat.
Remember that phrase
SHOOT TO STOP HIM. You aren't judge, jury, and executioner, you are simply saving your life and your wife's life. As soon as the threat is stopped, you stop.
When the police come, you say "I SHOT HIM TO STOP THE ATTACK I AM IN SHOCK I NEED MY LAWYER"
if they try and say 'talk to us first' continue to say "I AM IN SHOCK I NEED A LAWYER"
As far as what gun, a shotgun.
A shotgun is MUCH more powerful than any handgun. A shotgun is usually less expensive too.
There are two basic choices, 12 guage or 20 guage. 20 will kick less and is slightly less powerful but in the end it is more than powerful enough to get the job done. A 20 guage is more powerful than a 44 magnum revolver like dirty harry used.
On the economy end, you have two basic 'types', pump action and single shot.
Single shot is cheapest, and simplest. You can easily keep the shotgun open and under your bed, and the shells locked in a small lockbox. In emergency, open gun, insert shell, close gun, ready for use. Of course, one shot might not be great if you miss.
http://www.walmart.com/catalog/product.do?product_id=4665491
here is an example of a single shot. NEF (New England Firearms) or H&R (Harrington and Richards?) are the two main brands, and the guns sell for a little over $100
The other option is pump. Now, you do NOT want a pistol grip on the pump, even if the movies and TV says it is cool, it doesn't work in reality.
The shorter the barrel the better (makes it getting around the house easier) but that isn't a huge deal. Barrel length does nothing to performance of the gun beyond how easy it is to move around with it.
the cheapest decent pump for those on a tight budget is going to be the NEF pump shotgun
http://www.walmart.com/catalog/product.do?product_id=4665491
The other names to look at for pump shotguns at a reasonable price are
Charles Daly, Remingtion 870 Express, and Mossberg 500
All are availible in 20 guage or 12 guage, depending on which you choose.
Round this out with a few boxes of buckshot and you will have covered the 'hardware' part of the equation.
next is 'software' which is learning how to use it, when to use it, how to use it most effectively, etc, how to keep from needing to use it in the first place, etc etc etc. We can deal with that later, but just because you own a gun doesn't mean you know how to use it.