Unfortunately, there is no "one gun" for carry, particularly if you wear alot of casual clothes. There is always a trade-off between concealability, accuracy and stopping power. One option that I like when I'm carrying in the summer months and I'm dressed in shorts and a tee shirt is the fanny pack holster. Many of of them do tend to scream, "I'm carrying a concealed firearm!" to those who know what to look for,
but that may not be such a bad thing! The nice part about that it is you can carry one compact pistol in a "real caliber" such as 9mm up to .45 without too much difficulty. I've carried everything from a Beretta Tomcat to a 1911 Delta and I can tell you that weight and size matter! The bigger to absolute worse to carrry.
Personally, I'm a fan of the .40 S&W since it is nearly ballistically equivalent to the .45 ACP and can be had in much smaller packages. If I only had 1 gun, I'd set the minimal caliber at 9mm, to ensure at least marginal stopping power. I think for concealability, autos are really the way to go. Once I determined the caliber I wanted then I'd decide based on the size of the package that was available and what would be the best gun I could afford with certain features. I STRONGLY reccomend night sights, since the vast majority of shootings occur in low light conditions. I'd certainly consider good used pistols if they were in my price range and do not discount the likes of Taurus or especially CZ. Although the finish work on CZ pistols is usually a bit lacking for my tastes, they have one of the best out-of-the-box triggers I've sampled.
Although ALOT of people love Glocks and they are certainly proven, I just cannot get used to that funky trigger safety. But that probably won't be as big a deal for you since you're less likely to be set in your ways regarding trigger pull. I have to say that for $600, the new Taurus 1911 looks pretty damn nice, and I believe they have a Commander size version available. Kahr certainly makes a fine pistol, but they can be a bit pricey. One thing I would certainly do is scour
www.gunbroker.com for a good look at the market and some really competative pricing. This usually works quite well for more expensive pistols, because the cost of overnight shipping and transfer fee is often not that big a deal if you are saving $200-$400 on a high end pistol, but is not really worth it for less expensive guns. As for me, I've been carrying a stock H&K P2000 in .40 S&W for the past 2 years.