Well, I personally wouldnt want a mainland as i dont want anything over 10 feet. That might be something you want to keep in mind. That is why i went with a SD although Caesar isnt so SD really lol. He is between 7-8 feet or maybe even a little longer. i havent bothered to measure him.
I also would start him off in a big cage, at least a 4x2 but more like a 6x3. Retics arent too shy and just give him a few hides as a baby and he will be fine. They grow insanely fast, so a 20 gallon tank will last you maybe 6 months if you are lucky. Caesar was about 2 feet at 5 months old.
As for food, each retic will be different. Some will push if not fed, some are mellow. Caesar will be 2 years old this May and he eats either a jumbo rat or XL guinea pig every 10 days or so.
Also retics seem to need a lot more maintenance as they seem to love to piss and poop. Also they seem to get in a lot more trouble lol. I just had to slide Caesar's upper lip off a tooth as he somehow bit through his upper lip while grabbing his rat. It was not fun for either of us.
Retics are awesome but they take much more dedication on your part than with something like a ball python. They are also a lot more expensive in care.