For a pure HD gun, if you're concerned about spending too much money, my suggestion would be to go for a LMT 14.5" M4gery with a permanently pinned Phantom, i.e. a short, easy to aquire, Title 1 legal weapon, and an LMT cut carry handle fixed back up iron sight. For a light, I'd suggest the Insight Streamlight M3X Long Gun, and a mounting solution. The tape switch version will be better for most HD purposes. Decide where to put it based on your own comfort, but ensure that it's repeatable under the worst of conditions.
The beefed up light of the M3X is supposed to have throw and output that rivals Surefire's lights, at a lower price. I have the standard M3 version, but I'd like to get my hands on one of the M3X's, so I will qualify that. However, many military units are issuing M3X's rather than Surefires, and a version of that light is intended as the new visbile lighting system for the SOPMOD Block II program. I've also got a Surefire, and for tactical purposes, I won't play favorites between the two, but for simple HD, I would pick the Streamlight.
That being said, more specifics than that will differ greatly based on your own situation, budget, and desires. For example, if you've got a large amount of property to cover, you may not need an CQB optic, for distance = time. Just get some night sights, and drop 'em in. On the other hand, if you live in a single bedroom apartment, where, by the time an intruder is in your door, they've already closed the distance between you to twenty feet, you might want a reflex or CQB sight with long battery life so it can be kept on at night, or possibly even a visible laser. Considerations like these are going to be very important in determining your choices for a dedicated home defense rig.
I will most likely never have a dedicated HD carbine, I use them to train, and they pull double duty as HD. The HD carbine I described to you does not resemble any of mine, but it is what I would build if HD was the only thing that interested me, and I could not afford an optic.
As the others have said, a bright white light is going to be non-negotiable, you an choose almost any other combination of anything else, and argue forever on ARFCOM about it, but a solid light, the advantages of which have already been described, will be essential. If your HD rig ends up being a full length clone of an M16A1, so be it, as long as you've got a visible bright light.
Again, though, I would say solid, and standard iron sights, leave the FSB, and an LMT CCH BUIS, change the sights out for tritium, if you feel the need. The Phantom on the end of the 14.5" will not only let you get as short as possible, but the Phantom is well known to have great muzzle flash reducing capabilities. I would bolt a rail segment to the handguards, and mount a solid VFG, as well, and an ITI M3X long gun with remote tape switch.
Yes, there are other better options out there, some might suggest Noveske brakes, and the like, or SBR's or suppressors, but I got the impression that you don't want to spend that much for an optic, much less for a tax stamp, and exotic muzzle device.
You may later though...
HTH,
~Augee