I have a MSc and BA Hons in Business and Marketing and I have always liked the idea of teaching. However I would like to teach Linguistics and Language at University. I have identified a subject which crosses over (i.e. linguistics/language within advertising practices). I have been told by a lecturer I know that it doesn't matter what PhD I get, just go for one in Linguistics and I will be able to teach it! I wanted to know what people's thoughts are on this?

1) Will a PhD in Linguistics be sufficient to teach the subject? i.e. does the subject knowledge come from the literature review enough to teach it.
2) Will it be a big stretch for me to do a PhD in this subject?
3) Will it disadvantage me applying for jobs if I have a MSc and BA Hons in a separate subject?
4) Would it better to continue with my marketing background and do a PhD in this/continue to teach this area? I do like consumer behaviour so could find a PhD subject there.

Thank you in advance!

The best place to get an answer to this, if not from a Linguistics prof, is a career advisor at the university you are considering. Maybe look at the skill specifications of any current Linguistic jobs being advertised.
Your BA and MSc won't disadvantage you if you then have a PhD in Linguistics. But it also sounds like you aren't sure which path you want for your career and if that comes through during an interview you could be passed over for someone who presents as very committed and can give a good answer to the 'where do you see yourself in 5/10 years' question.