HELP ME PLEASE! UK visa

Hello,

I really need your help, I hope you can help me.

I am from Colombia, I have B1 english certificate and I am 19 years old. I have found a UK family on this amazing web. We have talked for skype 4 times and they have dicided they want me and I want them like me host family. I have read, issue visa for poeple who are not from EU needs tier 4 general student and requeriments are choose one of these courses:

Full-time and giving a qualification that’s at least level 6
Full-time, with at least 15 hours per week of daytime study and leading to qualification that’s at least level 3
Overseas, equal to UK higher education course and being run by an overseas institution

I think "Full-time, with at least 15 hours per week of daytime study and leading to qualification that’s at least level 3"  is the best option.

So here is my question, it says I have to prove that I have enough money to pay for the course and enough money for living costs as well, if my host family signs me invitation letter and au pair contract, do I have to prove I have enough money to living anyway? and is it really possible I can go to UK and work like an AU Pair?

I am waiting for your answer, I hope you can help me,

Thank you so much.