You are probably going to get a lot of people suggesting 1) pump over semi-auto for reliabilty (debateable) and cost (pumps are clearly cheaper) and 2) 12 ga over 20 ga because of the better ammo selection and availability and sometimes lower cost of 12 ga vs. 20 ga ammo.
The Remington site does not list a factory 18.5 inch barrel for the 1100, the closest is a 21" deer barrel with rifle sights and imp cyl choke. Perhaps someone else does offer them, though? You can order one for the 11-87 as those are also used by police. I don't know if you can switch gauges, rather than just barrels, and I am sure it would not be very economical even if you can. There is no need to change the magazine tube for capacity--three or four rounds is plenty for general home defense.
The big American gunmakers do make security semi-autos, but they are not cheap, and I can't speak to the advantages of one over the other. Ammo selection is more critical with semi-autos since a pump can use very light or very heavy loads without worrying about how it will cycle the action. One import brand I have heard good things about is Stoeger, but I have no personal experience with their semi-autos.
For a dedicated home defense gun, I would also consider the handiness and simplicy of a traditional double barrel--the Stoeger Coach gun, for example, in 12 ga, 20 ga or .410. Despite the 1.5" longer barrel vs. a Mossberg 500, the double is 3" shorter and loaded weight is about the same.
Good luck!
