70-300 is a good starting compromise, as long as you're not planning evening or indoor events then you don't need expensive f/2.8 lenses.
Not having image stabilisation isn't a big issue for moving subjects, so as a starter a cheap lens can do fine.

I have to admit that I don't have the entrepreneurial side, so if you want to make a living from this Simon's right - you have to find a way make it a business too.
For me that would mean finding a businessman to do that bit