I'm about the same age, I'm also circumcised, and I have the same darker discoloration on the lower part of my penis, just as you described. Its been this way since as long as I could remember. At least since puberty anyway. What it is, I have no idea, but I know its normal. I don't know if it has to do with the circumcision or not, since it is below where the circumcision scar is. But I've never seen this discoloration on uncut guys.
As for your rash and odor on your thigh, that's something entirely different an unrelated. I'm not sure what that is other than perhaps a case of a fungal infection such as jock itch. Try an anti-fungal ointment on the area - there are some ointments specifically for jock itch.