I would go with a standard Remington 870 and get a magazine extension tube later if you wish. You can get these at almost any Wal-Mart, Dicks, Sports Authority, or of course your local gun shop. They usually run around $300. These have choke tubes that you can change out depending on the situation (skeet, trap, turkey hunting, deer hunting, etc). The reason why I like the one with the wood stock or Mossy Oak cammo is that it looks like a hunting/sporting arm and is less "mean looking" to the officer that arrives to your house if an incident has happened and you have proceed to welcome the intruder to the full extent.
It all depends what you are wanting to use the firearm for, if you want a mean machine that looks like something that the army uses then by all means go and buy one. If you want something that can become more multipurpose, then I would recommend a basic Remington 870.
REMEMBER: Read your federal, state, and local gun laws often, get to know these laws. Know when you are allowed by law to shoot and when you are not allowed to shoot. Yes I realize that under stress of the situation you may and probably will forget or you will not agree with these laws, but they are the law and ignorance of the law is no excuse when the judge is in front of you. Do not shoot the intruder in the back, this will show the court/cops that the invader was trying to escape (stupid law/rule I know). If you are going to take the shot, always shoot square in the chest or in the side. NEVER IN THE BACK. Remember that the chest is a much bigger target than the head, groin, or wherever you might think a good area to shoot is.
Get good ammunition for your firearm, remember with shotgun if the metal on the shell is silver, DO NOT COUNT YOUR LIFE ON IT! This is some mystery metal to me (maybe Zinc) and is used in the cheap bulk value packs of shotgun shells. These are great for practice because they are cheap (100 rounds for 20 bucks). Always do a double check with the ammunition before you buy it, make sure that the metal is yellow (brass) and looks very fresh and shinny. Get 00 buckshot (has 9 .33 Cal balls) and some slugs (around .65 Cal I think). This is how I load my firearm in firing order.
1. Buckshot
2. Buckshot
3. Buckshot
4. Slug
If push comes to shove and the three rounds of buckshot do not take care of the life threat, the slug will finish it.
I do not keep a round chambered for safety, also so you can give the intruder an audio que of a shotgun cocking (classic chick-chick sound) to let them know that you mean business. Some people have other ideas on this, that it might give away your position. Ive heard may cases where this can actually scare off the common teenage crook that is looking to steal for some chump change. Be sure that you have a system with your family so you will know if its a friend or a foe. My family uses "IDENTIFY!!" shouted loudly and clearly, and the person is to respond immediately with his/her name loudly and clearly.
You can skip/hold off on toys for your shotgun (fancy mounted shell holders, extension tubes, heat shields, etc). They won't do much to save your life in a sticky situation. Mostly they are cosmetic and cops/judges/federal government hates them. The two things I would recommend is a cheap elastic shell holder that you can find at almost any sporting goods store including wal-mart in the fall. This will hold 5 or 6 extra rounds in case if you are a horrid shot or its a nasty firefight and need more ammo. And a flashlight that you can somehow attach to your firearm. The easiest thing to do is get one of those magazine tube stabilizers that you can get for cheap and put a cheap flashlight in it. This way you can have a light on the firearm and both hands are free to operate it. Here is an example of what i'm talking about...
http://www.cheaperthandirt.com/LIGHT121-1.html
And here is one of those shell holders (I have this one)
http://www.sportsmansguide.com/net/cb/cb.aspx?a=408027
When you get your shotgun, what ever you decide on, make sure you know how to operate it with your eyes closed. Every night and every morning load and unload the firearm a few times so that you know how it works in all situations. Get a box of those value rounds (20 bucks for 100 rounds) with that cheap metal so you can practice at the range for cheap. If they jam, no biggy, place the butt on your thigh and pull down in a fast motion on the foregrip. This will eject the jammed shell every time (i've found its harder to do with Mossberg shotguns over Remington 870s).
If you have any other questions, which I'm sure you will, don't hesitate to ask!