Remington has an 870 package with 20 in bbl and a mag extension. Has been a year or so since I bought mine, paid 350 for it new. HIGHLY recommend it, reliable and will handle anything you put through it. You can also pick up dirt cheap used 870s and add the mods yourself, it is an easy thing to do.
Also, your choice, but I recommend no red dot, just go with the old bead sight or a set of Wilson Ghost Rings with tritium. With just a bead sight I can hit a man sil at 100 yd with a low recoil slug with some kentucky windage, if I can you will have no problem inside close quarters. Concur with our comrades, red dot is just something else that can go wrong and you really dont need it. Good luck, bottom line is personal preference and what you can shoot well. regards