Fit your own double glazing?

I want to replace the 3 aging windows that make up a bay window in my
living room. The room is in a flat and is one up... for this reason I
would like to know if it is possible to do the job myself and from
INSIDE the flat. I presume that I would need to have windows made to my
spec and that they come with the glazing removed. Questions that spring
to mind...

Will the glazing be fitted from inside or outside?
How do you measure up for the windows?
Can I get brown upvc on the outside and white on the inside?
Rough cost for windows (1x 1.5x1.5 m & 2x 1.5x0.75 m)

TIA