It's really about what you like best. Seems like all your options are pretty good! I bought all except two of my BPs as babies and really enjoy them while they are young. I still love my adult BPs, however, it is a fun part of the experience to watch them grow from little worms into big snakes over several years.
As to your last question, it takes females a bit longer to be ready for breeding than males. Typically the recommendation for a female is to have made it past her third winter and weigh at least 1500 grams. My adult BP is right at 3 years old and exactly 1500 grams, so she is pretty much ready for her first breeding. Males can sometimes be good to go within a year. I have a 1 year old male BP who is already almost 900 grams and ready to mate.
P.S. I like the firefly! That would be my choice.
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