I'll first agree with most of the themes other posters have stressed:
- Keep it simple
- Develop your skills - practice practice practice
AIP Tactical is a good source of information. I spoke to J.D. last year when discussing a home defense shotgun; he's an experienced armorer and veteran, and had loads of good advice.
You haven't stated what you want the combat shotgun for, so I'll chuck in a couple points from a "home defense" viewpoint. If this is more of a "I want one for my collection", these points may not apply:
- A 20 gauge is going to do just fine in a home defense scenario - plenty of punch, and more controllable.
- Regularly get out your shotgun, shoot it, and regularly maintain it so you know it works when you really need it. Don't just toss it into the closet and forget about it - be a heck of a thing to reach for it and find your wife moved it to hang up clothes.
- Keep it simple.
- If you can get by with a good quality recoil pad rather than a fancy recoil reduction stock, do so.
- No fancy sights; front bead is fine
Take a look at
J.D.'s site; you can see examples of what he puts together, and either use them for a guideline or have him build one for you.