If I may throw in my .o2 on some of your questions.
First of all, delay any sort of customizing plan you have for a shotgun and/or a rifle. Given that you're just starting out, there's a shitload of info that you never even knew existed out there & meanwhile you may have already spent a ton of $$ on something you no longer think is the cat's ass.
The Mossberg 500 or 590 is a fine little shotgun for a pump. You could easily spend twice the cost of the gun on accessories based on what's out there now. Skimp on the accessories & instead spend it on ammo & range fees, IMO. After a few thousand rds down the barrel, you'll have a better idea of what would be better to have & (more importantly) what's NOT important.
A good source for being able to determine what you options are is to attend a local 3-Gun Match in your area. These matches comprise of a handgun, rifle & shotgun. You'll see most of the major brands there & in a variety of configurations as well.
WATCH THEM SHOOT A MATCH/STAGE! As Yogi Berra once said, "You can observe a lot by watching." Watch what equipment they use, what works, & on occassion, what doesn't work. Talk to them between stages or after a match, ask them why they chose Remington over a Benelli or vice versa. Most will be happy to tell why their brand is better than the other.
Be advised that Remington has a separate web site for their tactical shotguns from their duck hunter versions so as to not offend the sensibilities of the duck hunting crowd. Check that site for options as well.
As for your rifle, ask yourself, why you need a "long range tactical rifle" instead of an AR type? Keep in mind, you can swap out the upper of an AR between a bull barrel flat-top with a 50 power scope with an 16" barrel with a red dot for up close & personal work & use the same lower for both. This saves you $$$$. In case you didn't know, it takes about 20 seconds to swap the two uppers.
Unless you live out in the desert, I doubt you'd ever need a shot of over 300 yards. Most likely, half that & an AR can easily do what you need at about half the price.
You might want to ignore the advice given in the gun rags as too often they're driven by who buys their ads that determines what guns they say you HAVE to have.
99% of any scenerio you might encounter in the US can be simulated by a stage at a 3-Gun match. A good sidearm (which you already have), a good light rifle such as an AR (or FAL or even AK) & a good tactical shotgun are really all one would need to herd the demons in our society, IMO.
My .o2