hi there
right, so what are the walls, matt, silk, or eggshell, or even more lucky the washable stuff intended for kitchens and bahrooms.??
if they are matt, you are pretty well stuffed, as it will come off.
the ceilings, are they smooth or artex ??
your not going to completely clear this place, as houses are really constructed to achieve that, we have cleaned medical instrument wash rooms, where the whole place is cleaned top to bottom, and the air pressure in the room is set at greater to that of the surrounding rooms, with double airlock doors, so that there isnt any chance anything nasty getting into the room.
as soon as you open the front door of a house there is an effective air change, and therefore nasties can come into the house as well as leaving the house.
you really need to establish exactly what the client wants to achieve, one method would be to swab the place before you start, then have it tested, and then re swab after the clean to prove the cleanliness, but it will cost you and therefore the client money.
good luck
regards
martin