"mex mex" and "san luis potosi" are the same. They are part of the complex mexicana group which at one time also included the gray banded king snakes. You will see them sometimes as " variable kings". San Luis Potosi being the mexicana phase, there is also the "thayeri" and "greeri" phase. There are many color morphs in this group and as a rule are small docile king snakes, usually about 3' is tops. Now as for your questions, it depends on what you mean by "Normal king" To me that would be a banded Cal King or a Florida King, which should sell for $25-40 when they are hatchlings. You can usually pick up some of the variables for slightly more. What you should look for in a king, attentiveness, the snake should be actively moving around when you hold him, not limp and excessively calm in your hands(like any other snake) make sure the vent is clean, no lumps, scars etc. I have puchased kings of unknown age without any problems, price may be a factor with that question. Most of my Kings change color drastically when in the blue, don't know where that info came from. I have noticed no behavioral differences between the sexes, kings are generally not aggressive, but just like corns, there's one in every crowd. I hope this helped you a little. I love kingsnakes as much as I do corns. San Luis Potosi's can be really pretty, usually red,black and grey. If you get him post a pic here in the general chit chat forum.
Regards
Mike B