I'm going to answer as many of your questions as I can, but some of the questions don't have a defined answer and are really questions you should be asking and reflecting on yourself, rather than relying on a forum to tell you.
How fast can I progress through the pay scales when qualified (business studies)?
Depends where you work and how quickly you develop/take on additional responsibilities. With academisation, I get the impression this answer is a bit less clear than it used to be.
Will I be able to teach in secondary schools with my QTLS and how easy is it to achieve QTLS?
Yes, as I am sure you are aware, QTLS has parity with QTS. From talking to tutors, QTLS seems fairly straight forward to achieve.
Are the long hours/high stress worth the comparably smaller salary?
This is completely personal and something you should really ask yourself before you pursue this endeavour. Teaching salaries aren't great and it is hard work, but the rewards are also there.
Working in the Further education sector, is this a crumbling sector ie. budget cuts, will I struggle to find a job after my qualification (2 years)?
I wouldn't presume to be able to predict the decisions of government over the next 2 years. You can teach in schools, though, so would be able to do that even if FE cuts continue.
It will take be 6 years from now to earn what I am earning now, if I am worried about money should I continue?
Given the amount you focus on it I don't know. I know a vast number of people who have become teachers and pay has not been the primary motivator for anyone. If you value money that much, teaching isn't the profession for you. Just think, in a year or two's time when you are stressed, tired, have a pile of marking and are earning loads less than you do now, are the feelings of enjoyment and fulfillment compensate for the fact that your bank account is significantly less coushy?
What are the real benefits of teaching, to inspire me to continue? What are the real negatives of teaching that would push me back to the private sector?
If you don't know the answers to these, you either haven't bothered researching enough or haven't done enough placements. A lot of these are fairly self-evident if you read about teaching or have done a lot of placements. If you are already thinking you might return to the private sector, I'm not sure you are approaching this with the right mindset...
Is the course I am doing PGCE- Post compulsory sector really worth it, is it better just to do a PGCE or teach first and do it full time in a year? I guess it is almost like doing a 3 year pgce/NQT year?
No, it isn't. If I had the choice for my course, I would hands-down do a 'normal' PGCE (one that imbues QTS). It is more widely recognised and would enable you to work in schools without anyone questioning your ability to do so.
I want to reach the top of teaching deputy/headmaster, is this a realistic expectation with hard work and talent?
No idea, but given that people are headmasters/headmistresses, I would presume so.
I think my biggest concern is working for 2 years for a low salary to come out with a qualification in the further education sector which isnt relevant to secondary teaching, struggle to find a job, and then have a low salary for the next few years until I can reach at least 30k a year.
It'll be relevant, you are teaching 14+ students normally on a PGCE PCET, so you will gain experience. Particularly in business where some schools may only start it at GCSE anyway. Again, you shouldn't be doing this switch for the salary, teaching just isn't a job where you are going to be earning great money. Though, there are jobs abroad where you can make a lot of money. One of my old teachers got offered 70k, plus accommodation, tax free somewhere in the ME. If you are that desperate for money, maybe something like that would suit you.
On the other hand, I hate being stuck behind a desk in an office, I have a business degree. There are no bursaries for Business Studies teachers and my only other realistic option is Teach First, which, I am not interested in. I have had an experience day in a secondary school and I have 3 days more experience days coming up in a secondary school. I feel like for me teaching is now or never, but should I experience more time in the private sector (even though it is dull and meaningless).
Why is your only other realistic option teach first? Have you not looked at other methods of doing a pgce such as the salaried option? Also, could you not take the hit and just live off savings and student finance for a year? Teaching shouldn't be a now or never thing, you can do a pgce whenever. If you wanted to try and different job/different area in the private sector, you could certainly do that and then try teaching.
If you feel like what you are doing now is "dull and meaningless" then teaching could certainly potentially fill that void. Given your focus on salary, however, teaching is not a 'have your cake and eat it' option.... you are going to make what a lot of people would consider 'good' money, but hardly 'great' money. Also, as it is a nationally scaled salary, living somewhere like Newcastle (like me!), would allow you to live a significantly better lifestyle than expensive places down south (e.g. Cambridge, Oxford) where you will earn the same salary but have a significantly higher cost of living.