Getting paid to keep fit is brilliant! :o) It's long hard hours though, you have to work in other people's leisure time.
Are you planning on working in a private gym, council run gym, self employed or any or all af the above?
I wouldn't shell out all that cash straight away, you can go on a gym instructor course and start from there, there used to be two main ones, YMCA and RSA, the two then became OCR, far as I know that's still the one to have. Add strings to your bow as you go, nutrition, exercise to music, zumba, aqua running, BTS courses, you can learn as you earn. I'd recommend the ETA and Gym Instructor courses are the most necessary to get your foot in the door....apart from passion, energy, enthusiasm and a joy in helping people improve their lives. And, No, 37 isn't too late to do it, I'd say bloody well go ahead, enjoy, exercise is the cure-all, prevent-all in society, you name it, there is NOTHING in life that can't be helped by some sort of physical activity, and the government are realising this and putting money into education and prevention rather than cure.
I've done it before, during and after having kids,sometimes working til 10pm then being back on shift at 7:15am, so it's hard if you don't have a lot of support, or if your kids are very young.
There may be some way of getting a grant to help you pay for retraining, I'll see if I can dig out any info. Good luck! :o)