Okay, if you only want to speak and listen simply that would not be a big problem , but point is you need to read chinese too. and i do not know if there is any book wrote in pinyin. Later you will find out, most of time, when chinese talk about chinese , that does not include of pinyin .

No, you don't HAVE TO learn any characters. If you learn via audio you even don't have to learn Pinyin. Kids also can talk but can't read or write. However, learning characters helps the remembering process and pinyin help with the pronunciation. So from this viewpoint alone it's already useful.

like you said u just want to speak in chinese so it's up to you. but as you progress in chinese it think you would find it better to at least recognize characters. there are so many chinese characters with the same pinyin and tone and just knowing them by pinyin would really be confusing. also it is very important to learn the tones too.

pinyin is the foundation, but still not enough

of course you have to learn chinese characters,ppl dont use pinyin to chat online,well,if you can learn very well,at least you can understand what ppl say when u have face-to- face conversation.