Recommend you consider the .357Mag, as you can use the target .38Spl loads to get familiar (without killing your budget). 6 inch barrel is good for range work, but later if you decide for a CPL, you night want to consider then a 4 inch.
Revolver mention are great also look at the S&W m19's and the Ruger Security Six (on the Ruger you might find a drawer puppy safe queen, that need a good home.
Best caliber is WHAT FEEL good to you. ( I'm comfortable with either the 9mm. 357Mag or .45.)
Good luck ask more questions.
Is there a local club or commercial range in your area, to help try differnt sidearms?