I built a shed and am about to paint it and need to get some advice.
The shed is built with hardiplank siding and wood trim, what I need to
know is what is the difference between the $12.00 a gallon latex paint
and the $30.00 paint. And is it worth spending the extra money and go
with the more expensive just for a shed?

Next thing I would like to know is what should I paint the treated
plywood floor with to give it a protective caoting?

I will be using a Wagner Power Painter-Pro airless painter if that
makes any difference with your advice.