It doesn't have to be in the same field but your supervisors will want to know you're sufficiently competent to complete a PhD in a different field. I did an interdisciplinary PhD and I have to say, the half of mine that I wasn't familiar with from my previous studies was a real struggle. It's not impossible, but picking up a new field at PhD level is difficult.
Finance side of things - most people would advise you not to do a PhD unless you have full funding. You can get a student loan if you're in England now (not sure about the other countries - sorry!) but I'm not sure how it works out with living costs etc. The max you can get is 25k and full year fees on a PhD will be about 4k a year, then you might have research costs etc.