There are all sorts of thing you can do to a Remmington 870 or Mossberg 500. But if you are going to mod it a lot you might as well buy a used one. As everyone said, both shotguns are fine out of the box. If you want to make it tactical (or tacticool depending on who you are talking to) then that is a different story. Just look at one of the custom made 870s by Wilson Combat:
Wilson combat even has a deal where they take a beat up Remmington 870 and fix it up. It is something I have thought about doing.
Here is a before shot:
And an after shot:
Used 870s and 500s can be had for cheap, especially if found at a pawn shop (or other place where haggling is appropriate). If you are going to mod it, you might as well buy a cheap used one. FYI, newer 870s have a detent in the magazine which prevents a magazine extension from being installed (may or may not be important to you).