The 642 is a great carry gun. I would not want to get rid of mine. I am carrying mine in either a Fobus Paddle (Dont bother folks to tell me it is junk since I have used it for close to 10 years) and an EP #88 "Street Combat" Holster. I got the #88 for my 640, but the 642 fits OK. I don't like the suede lining, and would not buy it again. It is not a bad holster, however, and you would probably be happy with it.
#88 "Street Combat" Holster
I would highly recommend Kevin's Eagle or Laredo Defender.
K&D Holsters | Custom Gun Leather and Shooting Accessories
I do not think you need any kind of retention device. If a holster holds a loaded gun upside down without falling out there will be no problem. Any good maker will provide one that is plenty tight.
For ammo, I practice with handloads, and carry both 158 gr LSWHP +P and Speer GD 135 gr +P ammunition.
Regards,
Jerry