I literally this weekend just made some upgrades to my Remington 870 that sound similar to what you're after for your Mossberg. A local dealer helped me select:
1. A CAA (Command Arms Accessories) model SGR1 Shotgun Rail Adaptor, which is basically a screw-on Picatinny rail that I mounted on the left side of the 870's barrel just ahead of the front-end of the slide (
www.commandarms.com).
2. A Surefire model M79 Flashlight Mount (
www.surefire.com); clamps securely around a Surefire or other 1" diameter flashlight.
3. A Surefire model 6P-LED flashlight in black...classic handheld light with lockout tailcap, etc., powered by two 123 lithium batteries.
I could have (and may later) add a pressure-switch to the light, but off-the-cuff I wasn't sure how/where to mount that given the slide-action nature of the shotgun. Total retail on the above was about $200.
I also purchased separately a black elastic buttstock shell holder that holds five shells. I also have a sling mounted, and it was a bit of a stretch to fit that shell holder over the rear sling swivel mounting point on the 870's synthetic stock. This item cost a mere $3.50!
While this all looks great and feels super-solid, I'll be "proving" it my next trip to the range.
Not being very Web-proficient, I haven't mastered posting online pics, but I could email you some if you'd PM me with your email address.
Hope this helps!