The easiest way to accurately date without contacting Remington is by the barrel code located on the left side of the barrel just in front of the receiver. It should be two letters, for example KR. The first letter is month, the second is the year.
The serial number prefix ‘B’ fits within a 3-4 year range from 1994 to 1997 according to the notes in my armorer’s manual. The ‘M’ denotes that it is a Magnum Receiver.
I believe that Remington’s QC in the 90’s was good enough I’d trust it. If you’ve fired it very much the action should be smooth. The lack of detent balls on the barrels is no big deal if you are using the wave washer with the Wilson extended tube. I’d get a new Wilson spring and a new wave washer if it has been sitting loaded for an extended period.
The heavier carrier dog spring is not that big of a deal to me, I’ve used 870’s since the mid 70’s, the first police shotguns I used were merely Wingmasters with 18 or 20 inch barrels. Remember, Remington has sold slug guns for hunters for years without the heavier carrier dog spring, so like I said, I wouldn’t be too worried about replacing it. If you decide you want to here is a source - http://www.midwayusa.com/product/129237/remington-carrier-latch-spring-1100.
Likewise the sear spring is a matter of preference, it runs you up to 5-7 pounds, here is a source - http://www.midwayusa.com/product/440264/remington-sear-spring-870-heavy.
I think you could replace them yourself if you want. If you get them, I or someone else can walk you through it.
The next thing is the forearm, I assume you might want to carry additional ammo, and for my money a sidesaddle is the best way to do this. TacStar makes a 4 pack side saddle that will fit with your hunting forearm. It is pricey - http://www.tacstar.com/home/4-shot.php - you can maybe get it cheaper elsewhere. I have a fleet of 28 training shotguns, we have TacStars on everyone of them, they are constantly stripping threads in the aluminum plates. For your use, they’d probably be fine, for constant use not so great IMO.
The other option is velcro - this is what I have on my personal shotguns - http://www.skdtac.com/Esstac-Shotgun-Card-p/ess.106.htm - cheaper and if you have a couple you can just slap them on. I haven’t done this yet, but I’m thinking you could take a box cutter with a fresh blade and cut it down to fit with the longer forearm.
For a light mount here is what I’d do - http://www.gggaz.com/remington-870-sling-and-flashlight-combo-mount.html - then you could either use a pistol WML or any other rail compatible mount. Not sure how it would interface with the Wilson, but since Wilson sells a sling mount of the same type, I’d bet it would work.
For HD weapons I don’t keep slings mounted so I’m not going to address that. My belief is I’ve either put the guy down and am covering him, or I’ve emptied the weapon and either 1) thrown it at him or 2) clubbing him to death with it. YMMV.