My shop is the Canadian distributor for Kimber, and I shoot a Custom CDP II as my IDPA gun, and you'll fine no finer bang for your buck anywhere IMO if you're looking for a .45 as your defensive system.
I strongly recommend taking a weekend course on home defence, which will teach you not only the proper use of a firearm in an indoor environment, but also teach you how to set up your house and have fall back/lockdown positions with phones in the right places, etc.
Also, caliber IMO is not as big an issue as the type of cartridge you use. Full metal jacket is a major no no, be sure you use a round specifically made for defence purposes (ie kill people, not put holes in paper.)
Unlike what many are saying here, 9mm with 147 gr hydrashocks or other similar rounds don't overpenetrate at all, in fact they often penetrate even less than a similar 230 gr .45 hollowpoint/frag round. .45 is a lot of gun for a newbie, 9mm is easier to shoot, usually smaller, cheaper on ammo, and will suit your purpose just fine, IF you use the correct ammunition for the purpose.
I just took a course from SigArms last weekend on advanced tactical pistol, Tim Connell who trains the Air Marshalls as well as "other" military units in the art of pistelro was up for a week or so, and he said unofficially that the Air Marshalls are using Sig's with the .357 Sig cartridge, which is virtually the same as any 9 in caliber and weight, only going about 400 fps faster. They obviously have the ammunition/penetration situation well in hand there, being on aircraft and all, so don't worry about a 9mm going through your house and into your neighbours. It can happen with any caliber, if full metal jacket or lead swc type ammo is used.
Best advice that I've read here and can give is to seek the help of a local professional, and not be afraid of trying out what he suggests.