What size exo terra viv should I get?

I am going to get a juvie crested gecko and I was wondering which size viv I should get. I was planning on getting the 12",12",18" and then upgrading to the 45cm,45cm,60cm one after about a year. would this be ok? I was just thinking about him finding his food. and also if it was better to buy the rainforest kit, or just the viv and buy all the accessories separately. What do you all think???