both are natural but I would say the first.

Yes, both are quite natural.

Thanks Hayley for your suggestions!

This may be a regional thing, but i would replace the "this" for "that". I live in the North West of England and a lot of people do say: "Don't get me started on that. I will be talking for hours" "Don't let me get started on that, I will talk for hours" Both would infer that the person is very passionate on the subject that has been brought up. For example; Person A: "Have you heard about the police on the news?" Person B: "Don't get me started on that. I will be talking for hours!" or of course, "Don't let me get started on that, I will talk for hours!".