You can build on iOS devices without worrying about the $99/year fee, if you jailbreak them. I would say that Android would probably be the best way for you to go though since you already have some Java experience and only have an old Macbook.
That is one of my future goals to get to the point to develop iOS apps. But first I need to do Android because it does not have a developer tax and has better tools to write apps. I haven't figured out XCode and Objective-C yet, but I have a better understanding of Java because I learned C in 1987 and then Java and other languages later because my knowledge of C helped me learn the others.
When I was young, my father worked for AT&T and during a Pioneer meeting I once met Dennis Ritche who told me to learn programming. That is why in 1987 I took the C language class, he had a hand in developing it.