I have seen some really great advice posted in the threads previous to mine on species that fit your criteria. But I think the best advice that can be given to you is to do some research into the different snakes available in the herp trade. Buy a book that deals with the different genera. There are several available with loads of information. Because, at the end of the day, you are the one cleaning up the snake poop for the next 25 - 40 years.