Joey, I'll put in a good word for Hans Vang. You'll never deal with a nicer guy or better company, IMHO. I've dealt with Robar as well, and they're good folks. I don't have any experience dealing with Scattergun, but I did call them before I bought, and they seemed fairly helpful as well.
Hans is a shotgun expert, and to my knowledge, this is pretty much all he does. If you go the whole nine yards, including 18" ported barrel, SureFire foreend, magazine extension (with high-visibility follower), sidesaddle, MMC sights, Giles sling, etc., then it will set you back about $1,200 as I recall. But, send an email to Hans and Bill (his sidekick), and they'll give you specific pricing and suggestions. Or call, and speak to Hans directly. I can tell you from personal experience that he is very 'user-friendly' when it comes to folks that have an 870, but aren't exactly sure what changes they want or need.
At a minimum, I would respectfully suggest you consider an 18" barrel (bead sight, cylinder bore - Remington probably best, but Mossberg barrel's are available as well), mag extension, sidesaddle, and Giles sling. This gives you the basics at a reasonable price (approximately $300 to $400, as I recall), and turns that hunting shotgun into much more of a defensive shotgun. If you can stand the expense, add the Vang Comp ported barrel with MMC sights (consider tritium in the front as well). Or, simply add that later when it makes sense financially. The SureFire foreend is clearly not only a matter of expense (high), but also personal preference and a debate in and of itself.
Call Hans before you decide. He is quite a guy, and again, one of the most helpful fellows I've found in the firearms business.
I haven't been in the 870 market lately, but move fast - I hear they are becoming scarce, and I would assume that may translate to the parts as well. On the other hand, those rumors may simply be more Y2K hype, I don't know.
Good luck, and regards from AZ.
[This message has been edited by Jeff Thomas (edited August 09, 1999).]