Marked walls

[Newapprentice]

I've been asked to paint the inside of a shop; white emulsion onto white emulsion. The walls have lots of scuffs and marks from peoples shoes over the years, should I rub the walls down with sandpaper, then wash with sugar soap, should I paint a couple of coats of stain block over the bad areas?

Also, the walls are quite large, I was going to use my normal 9" roller and long pole, I believe there are large rollers out there, is it worth using something bigger or stick to what I know?

Any advise is very much appreciated.