In British English we would say 'I've got some thinking to do'/ 'I've got things to think about'. I've got some thinking to do = I have to think about what you said/what I have to do. I've got things to think about= I have different problems on my mind/ decisions to make etc

Yes it is mate

no no no i dont think so

Yes.

Yep. Unless you get reaaaaaally technical, and read way into the sentences (and depending on what context they're used in) they're really the same thing.