Not sure how big a Mex black can get but adult king snakes should average out btwn 3-6 feet (varies from Ssp to Ssp)... a snke can reach adulthood in as little as 18months but i'd say a snake would be very close to full grown at 3yrs. (i don't think they every really STOP growing... have no proof but...)
Housing
an adult king can be house in a 20 gal long enclosure but if you have space for bigger go bigger.
custom build or buy:
Depends on what you wanna do... I custom build cuz I have too many snakes to house each of them in aquariums but if you have one or two then I think buying an enclosure can work out fine
Fresh killed or Live:
I say fresh killed... your snake won't get hurt by a feisty mouse that won't go out without a fight.
Heat:
under tank for aquarium, heat tape for enclosure (heat rock are a big no no, heat lamps aren't necessary)
Hides
I've read that you can use two but I use one some times none, but I put the hide on the cooler side of the enclosures, if they get too cold they'll migrate to the warmer side.
Shedding:
snakes shed at least once a month, sometimes twice a month
one of my multilevel custom display enclosures: the other one is almost finished

hope that helps
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Thanx
Royal Blue Reptilez
~ZF