Why do you want to leave your field? Isn't there perhaps some specific need or niche that you can find within the sector you are working in? You might even be able to get your current employer to be your first customer, by solving a problem they already have. Just my 2c.

It's hard to start a business, but starting a business in what you already know and are familiar with makes it a little easier.

Anyway, sorry for not answering any of your questions, I just thought I'd prod you to think a bit more out of the box. Good luck with whatever course you decide upon!