errrmmm, dont get me wrong. Saving money is something we all do. Esp. these days. However, in my opinion, a rewire is a definate must. This is obviously an old installation so cables would have degraded a great deal. by the time you have done all the extended testing, sorted any problems. realised someone who has done an alteration at some point and has done something dodgy, sorted that, found the right screws, earthed the boxes, sorted any testing problems out, finding out that the customer wants another socket the other side of the room etc. the list can go on.
after all this, how much longer (and how much more money) would it have taken to just rewire the place?
You've gotta think safety as well, would you really want your name on this installtion and to be held accountable if something should go very wrong?