Hi Ben
You can add an FI rating to your PPL but you couldn't then charge anything for your services. I've never met anyone who hasn't done their CPL first (remember 150 hours needed for that) and full ATPL passes before you can do that.
In terms of pay... this is a long debate. I earn £20 for showing up and £22 per hour of flight (or supervising solo students whilst they are in flight and i'm drinking coffee). That is unusually high though. I reckon about £18 is average, with some schools paying peanuts. Also remember it's very seasonal. Lots of hours in the summer and few in the winter.
I love it, it IS great for hours building (I've done about 600 in nine months), you learn a lot about aeroplanes, people and yourself.
Anything else don't hesitate to message me.
METAR
It's all about the now!