I will tell you what I will do if I am to build a "Tac" shotgun.
1) What would you use as the base (ie Remington 870, Mossberg 550, Winchester 1200)
I will use Mossberg 590a1 series(models with metal trigger groups) over the Remington 870 and the Winchester 1200. Because to me the Mossberg is a better designed shotgun then the other two designs. Firstly, the Winchester is not being produce anymore, for I will not use a non-production model for my base. Secondly, the Remington 870 design is not as good as the Mossberg's. For example, the location of the Mossberg's safety and slide stop are at ergonomic locations, that is you can use them without moving your shooting hand(right hand) In contrast, the Remington 870 design requires the user to use two hands to use the safety and slide release. Lastly, how the shell elevator(a Mossberg term, aka shell lifter) cams UP back to the action is best thing about the Mossberg design, for a person can just dump in the shells and load them in the tube. Basically the Mossberg 500 series is superior due many smart features over the Remington design.
2) What would be the suggested length of the barrel?
I will go with 18.5 inches, so that the muzzle blasts are tolerable without hearing protections. Without hearing protections, any length shorter then 18 inches will be not tolerable for any "action" usages.
3) What furniture would you use?
Mossberg factory stock, without the speedfeed slots. I will not use the speedfeed stocks because they don't work well and cost more.
4) What sighting system (if any) would you install?
Personally, I really have no preference for what sights to use. For a shotgun, most likely a person will never need to use the sights to shoot a "thing". The best examples are the skeet and trap shooters when they shoot their shotguns.
5) What would be the magazine capacity?
I think that the more BBs you can get in any guns the better things will be.
6) What other accessories would you add?
I would get a good sling for easy transportation and quick transition. Next, you will need buckshots and more of them, and use them for training. Lastly, get some dummy shells that weight the same as real shells to practice reloading.